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Versatile, high-performance histogram toolkit for Numpy.
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README.rst

histbook

histbook

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Versatile, high-performance histogram toolkit for Numpy.

histbook computes histograms from Numpy arrays. It differs from most other histogramming tools in that its histograms are primarily tables of numbers, rather than display graphics. Histograms can be filled and refilled iteratively through a large dataset, or in parallel and later combined with addition*. Histograms have arbitrarily many dimensions with convenient methods for selecting, rebinning, and projecting into lower-dimensional spaces.

Axis dimensions are managed by algebraic expressions, rather than string labels or index positions, so they are computable: an axis named x + y requires two Numpy arrays, x and y, which will be added before filling the histogram. Expressions in different axes or different histograms in the same "book" (a collection of named histograms) are computed in an optimized way, reusing subexpressions wherever possible for quicker filling without giving up clarity.

Histogram data may be exported to a variety of formats, such as Pandas, ROOT, and HEPData. It can also be plotted with Vega-Lite, which makes short work of projecting many dimensions of data as overlays and trellises.

(*In this respect, histbook is like histogramming packages developed for particle physics, from CERN HBOOK in the 1970's (name similarity intended) to modern-day ROOT.)

Installation

Install histbook like any other Python package:

pip install histbook --user

or similar (use sudo, --user, virtualenv, or pip-in-conda if you wish).

Strict dependencies:

Recommended dependencies:

  • Pandas for more convenient programmatic access to bin contents
  • Jupyter Notebook for interlaced histogramming and plotting
  • JupyterLab for a complete IDE-like environment
  • ipyvega to view plots in a Jupyter Notebook (not needed for JupyterLab)
  • Altair to mix histograms with Altair graphics (usable in both Jupyter Notebook and JupyterLab)
  • VegaScope to view plots in a web browser without Jupyter
  • ROOT to analyze histograms in a complete statistical toolkit
  • uproot to access ROOT files without the full ROOT framework

Tutorial

Table of contents:

Interactive tutorial

Run this tutorial on Binder.

Reference documentation

Getting started

Install histbook and pandas. If you'll be using a Jupyter Notebook, install ipyvega. If you'll be using a bare Python terminal, install VegaScope. If you'll be using JupyterLab, no visualization library is necessary.

pip install histbook pandas vega vegascope --user          # "vega" is ipyvega

Then start your environment to get a Python prompt.

>>> from histbook import *
>>> import numpy

and create a canvas to draw Vega-Lite graphics, if necessary.

>>> from vega import VegaLite as canvas                    # for ipyvega in Jupyter Notebook
>>> import vegascope; canvas = vegascope.LocalCanvas()     # for VegaScope in bare Python
                                                           # JupyterLab doesn't need anything

Let's start by histogramming a simple array of data.

>>> array = numpy.random.normal(0, 1, 1000000)
>>> histogram = Hist(bin("data", 10, -5, 5))
>>> histogram.fill(data=array)
>>> histogram.step("data").to(canvas)                      # for JupyterLab, drop ".to(canvas)"

docs/source/intro-1.png

What just happened here?

  • The first line created a million-element Numpy array.
  • The second created a one-dimensional histogram, splitting data into 10 bins from −5 to 5.
  • The third line incremented histogram bins by counting the number of values that lie within each of the 10 subintervals.
  • The fourth line projected the hypercube onto steps in the data axis and passed the Vega-Lite visualization to canvas.

We could also access the data as a table, as a Pandas DataFrame:

>>> histogram.pandas()
               count()  err(count())
data
[-inf, -5.0)       0.0      0.000000
[-5.0, -4.0)      33.0      5.744563
[-4.0, -3.0)    1247.0     35.312887
[-3.0, -2.0)   21260.0    145.808093
[-2.0, -1.0)  136067.0    368.872607
[-1.0, 0.0)   341355.0    584.255937
[0.0, 1.0)    341143.0    584.074482
[1.0, 2.0)    136072.0    368.879384
[2.0, 3.0)     21474.0    146.540097
[3.0, 4.0)      1320.0     36.331804
[4.0, 5.0)        29.0      5.385165
[5.0, inf)         0.0      0.000000
{NaN}              0.0      0.000000

including underflow ([-inf, -5.0)), overflow ([5.0, inf)), and nanflow ({NaN}). In the absence of weights, the error in the count is the square root of the count (approximation of Poisson statistics; histbook makes the same statistical assumptions as ROOT).

This example was deliberately simple. We can extend the binning to two dimensions and use expressions in the axis labels, rather than simple names:

>>> import math
>>> hist = Hist(bin("sqrt(x**2 + y**2)", 5, 0, 1),
...             bin("arctan2(y, x)", 3, -math.pi, math.pi))
>>> hist.fill(x=numpy.random.normal(0, 1, 1000000),
...           y=numpy.random.normal(0, 1, 1000000))
>>> beside(hist.step("sqrt(y**2 + x**2)"), hist.step("arctan2(y,x)")).to(canvas)

docs/source/intro-2.png

Note that I defined the first axis as sqrt(x**2 + y**2) and then accessed it as sqrt(y**2 + x**2) (x and y are reversed). The text between quotation marks is not a label that must be matched exactly, it's a symbolic expression that is matched algebraically.

The data contained in the Hist is two-dimensional, which you can see by printing it as a Pandas table. (Pandas pretty-prints the nested indexes.)

>>> hist.pandas()
                                                    count()  err(count())
sqrt(x**2 + y**2) arctan2(y, x)
[-inf, 0.0)       [-inf, -3.14159265359)                0.0      0.000000
                  [-3.14159265359, -1.0471975512)       0.0      0.000000
                  [-1.0471975512, 1.0471975512)         0.0      0.000000
                  [1.0471975512, 3.14159265359)         0.0      0.000000
                  [3.14159265359, inf)                  0.0      0.000000
                  {NaN}                                 0.0      0.000000
[0.0, 0.2)        [-inf, -3.14159265359)                0.0      0.000000
                  [-3.14159265359, -1.0471975512)    6704.0     81.877958
                  [-1.0471975512, 1.0471975512)      6595.0     81.209605
                  [1.0471975512, 3.14159265359)      6409.0     80.056230
                  [3.14159265359, inf)                  0.0      0.000000
                  {NaN}                                 0.0      0.000000
[0.2, 0.4)        [-inf, -3.14159265359)                0.0      0.000000
                  [-3.14159265359, -1.0471975512)   19008.0    137.869504
                  [-1.0471975512, 1.0471975512)     19312.0    138.967622
                  [1.0471975512, 3.14159265359)     19137.0    138.336546
                  [3.14159265359, inf)                  0.0      0.000000
                  {NaN}                                 0.0      0.000000
[0.4, 0.6)        [-inf, -3.14159265359)                0.0      0.000000
                  [-3.14159265359, -1.0471975512)   29266.0    171.073084
                  [-1.0471975512, 1.0471975512)     29163.0    170.771778
                  [1.0471975512, 3.14159265359)     29293.0    171.151979
                  [3.14159265359, inf)                  0.0      0.000000
                  {NaN}                                 0.0      0.000000
[0.6, 0.8)        [-inf, -3.14159265359)                0.0      0.000000
                  [-3.14159265359, -1.0471975512)   36289.0    190.496719
                  [-1.0471975512, 1.0471975512)     36227.0    190.333917
                  [1.0471975512, 3.14159265359)     36145.0    190.118384
                  [3.14159265359, inf)                  0.0      0.000000
                  {NaN}                                 0.0      0.000000
[0.8, 1.0)        [-inf, -3.14159265359)                0.0      0.000000
                  [-3.14159265359, -1.0471975512)   39931.0    199.827426
                  [-1.0471975512, 1.0471975512)     39769.0    199.421664
                  [1.0471975512, 3.14159265359)     39752.0    199.379036
                  [3.14159265359, inf)                  0.0      0.000000
                  {NaN}                                 0.0      0.000000
[1.0, inf)        [-inf, -3.14159265359)                0.0      0.000000
                  [-3.14159265359, -1.0471975512)  202393.0    449.881095
                  [-1.0471975512, 1.0471975512)    202686.0    450.206619
                  [1.0471975512, 3.14159265359)    201921.0    449.356206
                  [3.14159265359, inf)                  0.0      0.000000
                  {NaN}                                 0.0      0.000000
{NaN}             [-inf, -3.14159265359)                0.0      0.000000
                  [-3.14159265359, -1.0471975512)       0.0      0.000000
                  [-1.0471975512, 1.0471975512)         0.0      0.000000
                  [1.0471975512, 3.14159265359)         0.0      0.000000
                  [3.14159265359, inf)                  0.0      0.000000
                  {NaN}                                 0.0      0.000000

With multiple dimensions, we can project it out different ways. The overlay method draws all the bins of one axis as separate lines in the projection of the other.

>>> hist.overlay("arctan2(y, x)").step("sqrt(x**2+y**2)").to(canvas)

docs/source/intro-3.png

The stack method draws them cumulatively, though it only works with area (filled) rendering.

>>> r = "sqrt(x**2 + y**2)"
>>> phi = "arctan2(y, x)"
>>> hist.stack(phi).area(r).to(canvas)

docs/source/intro-4.png

The underflow, overflow, and nanflow curves are empty. Let's exclude them with a post-aggregation selection. You can select at any bin boundary of any axis, as long as the inequalities match (e.g. <= for left edges and < for right edges for an axis with closedlow=True).

>>> hist.select("-pi <= arctan2(y, x) < pi").stack(phi).area(r).to(canvas)

docs/source/intro-5.png

We can also split side-by-side and top-down:

>>> hist.select("-pi <= arctan2(y, x) < pi").beside(phi).line(r).to(canvas)

docs/source/intro-6.png

>>> hist.select("-pi <= arctan2(y, x) < pi").below(phi).marker(r, error=False).to(canvas)

docs/source/intro-7.png

Notice that the three subfigures are labeled by their arctan2(y, x) bins. This "trellis plot" formed with beside and below separated data just as overlay and stack separated data. Using all but one together, we could visualize four dimensions at once:

>>> import random
>>> labels = "one", "two", "three"
>>> hist = Hist(groupby("a"),                     # categorical axis: distinct strings are bins
...             cut("b > 1"),                     # cut axis: two bins (pass and fail)
...             split("c", (-3, 0, 1, 2, 3)),     # non-uniformly split the data
...             bin("d", 50, -3, 3))              # uniform bins, conventional histogram
>>> hist.fill(a=[random.choice(labels) for i in range(1000000)],
...           b=numpy.random.normal(0, 1, 1000000),
...           c=numpy.random.normal(0, 1, 1000000),
...           d=numpy.random.normal(0, 1, 1000000))
>>> hist.beside("a").below("b > 1").overlay("c").step("d").to(canvas)

docs/source/intro-8.png

In the above, only the last line does any drawing. The syntax is deliberately succinct to encourage interactive exploration. For instance, you can quickly switch from plotting "c" side-by-side with "b > 1" as bars:

>>> hist.beside("c").bar("b > 1").to(canvas)

docs/source/intro-9.png

to plotting "b > 1" side-by-side with "c" as bars:

>>> hist.beside("b > 1").bar("c").to(canvas)

docs/source/intro-10.png

or rather, as an area:

>>> hist.beside("b > 1").area("c").to(canvas)

docs/source/intro-11.png

We see the same trend in different ways. Whatever axes are not mentioned are summed over: imagine a hypercube whose shadows you project onto the graphical elements of bars, areas, lines, overlays, and trellises.

Axis constructors

Histograms can be built from the following types of axis:

  • groupby(expr) to bin by unique values, usually strings or integers (categorical binning)
  • groupbin(expr, binwidth) to create new bins when they appear in the data (regularly spaced, sparse binning)
  • bin(expr, numbins, low, high) for a fixed number of bins in a given range (regularly spaced, dense binning)
  • intbin(expr, min, max) for integer-valued bins between min and max, inclusive (same as above, but for integers)
  • split(expr, edges) for a fixed number of bins between a set of given edges (irregularly spaced, dense binning)
  • cut(expr) to divide the data into entries that pass or fail a boolean predicate (two bins)
  • profile(expr) to collect the mean and error in the mean of a dependent variable (not binned)

Profile plots

We can profile "y" and "z" or as many distributions as we want in a single Hist object.

>>> x = numpy.random.normal(0, 1, 10000)
>>> y = x**2 + numpy.random.normal(0, 5, 10000)
>>> z = -x**3 + numpy.random.normal(0, 5, 10000)

>>> h = Hist(bin("x", 100, -5, 5), profile("y"), profile("z"))
>>> h.fill(x=x, y=y, z=z)
>>> beside(h.marker("x", "y"), h.marker("x", "z")).to(canvas)

docs/source/intro-12.png

>>> h.select("-1 <= x < 1").pandas("y", "z")
              count()  err(count())         y    err(y)         z    err(z)
x
[-1.0, -0.9)    243.0     15.588457  1.104575  0.319523  1.135648  0.301416
[-0.9, -0.8)    275.0     16.583124  0.775029  0.312829  0.485808  0.302074
[-0.8, -0.7)    317.0     17.804494  0.505641  0.300481  0.427452  0.274324
[-0.7, -0.6)    315.0     17.748239  0.358800  0.268928  0.823575  0.288089
[-0.6, -0.5)    351.0     18.734994  0.691492  0.262019 -0.081257  0.265111
[-0.5, -0.4)    359.0     18.947295  0.116491  0.263602  0.171423  0.273736
[-0.4, -0.3)    359.0     18.947295  0.349983  0.256635 -0.107522  0.262714
[-0.3, -0.2)    392.0     19.798990  0.060286  0.257601  0.203810  0.252574
[-0.2, -0.1)    369.0     19.209373  0.207661  0.246779  0.355550  0.268741
[-0.1, 0.0)     388.0     19.697716  0.111659  0.258635  0.223001  0.265828
[0.0, 0.1)      382.0     19.544820  0.348179  0.243986  0.292852  0.249558
[0.1, 0.2)      378.0     19.442222  0.332284  0.273607 -0.277728  0.248078
[0.2, 0.3)      401.0     20.024984  0.100446  0.241673 -0.052257  0.258555
[0.3, 0.4)      386.0     19.646883  0.356500  0.246703 -0.014357  0.251480
[0.4, 0.5)      369.0     19.209373  0.421627  0.258498 -0.073345  0.261555
[0.5, 0.6)      355.0     18.841444 -0.060199  0.259124 -0.383521  0.255889
[0.6, 0.7)      335.0     18.303005  0.560394  0.272651 -0.239575  0.287837
[0.7, 0.8)      298.0     17.262677  0.499264  0.264333 -0.453906  0.282144
[0.8, 0.9)      291.0     17.058722  1.449089  0.293750 -0.920633  0.306683
[0.9, 1.0)      267.0     16.340135  1.085551  0.287038 -1.120942  0.304403

Although each non-profile axis multiplies the number of bins and therefore its memory use, profiles merely add to the number of bins. In fact, they share some statistics, making it 33% (unweighted) to 50% (weighted) more efficient to combine profiles with the same binning. Perhaps more importantly, it's an organizational aid.

Weighted data

In addition to bins, Hist takes a weight parameter to compute weights for each input value. A value with weight 2 is roughly equivalent to having two values with all other attributes being equal (for counts, sums, and means, but not standard deviations). Weights may be zero or even negative.

For example: without weights, counts are integers and the effective counts (used for weighted profiles) are equal to the counts.

>>> x = numpy.random.normal(0, 1, 10000)
>>> y = x**2 + numpy.random.normal(0, 5, 10000)

>>> h = Hist(bin("x", 100, -5, 5), profile("y"))
>>> h.fill(x=x, y=y)
>>> h.select("-0.5 <= x < 0.5").pandas("y", effcount=True)
              count()  err(count())  effcount()         y    err(y)
x
[-0.5, -0.4)    381.0     19.519221       381.0  0.124497  0.251414
[-0.4, -0.3)    388.0     19.697716       388.0  0.215915  0.241851
[-0.3, -0.2)    376.0     19.390719       376.0 -0.029105  0.252925
[-0.2, -0.1)    410.0     20.248457       410.0 -0.128061  0.249327
[-0.1, 0.0)     392.0     19.798990       392.0  0.199057  0.250275
[0.0, 0.1)      398.0     19.949937       398.0 -0.081793  0.242204
[0.1, 0.2)      401.0     20.024984       401.0 -0.144345  0.258108
[0.2, 0.3)      397.0     19.924859       397.0  0.083175  0.251312
[0.3, 0.4)      381.0     19.519221       381.0  0.065216  0.248393
[0.4, 0.5)      341.0     18.466185       341.0  0.349919  0.267243

Below, we make the weights normal-distributed with a mean of 1 and a standard deviation of 4 (many of them are negative, but the average is 1). The counts are no longer integers, errors in the count are much larger, effective counts much smaller, and it affects the profile central values and errors.

>>> h = Hist(bin("x", 100, -5, 5), profile("y"), weight="w")
>>> h.fill(x=x, y=y, w=numpy.random.normal(1, 4, 10000))
>>> h.select("-0.5 <= x < 0.5").pandas("y", effcount=True)
                 count()  err(count())  effcount()         y    err(y)
x
[-0.5, -0.4)  310.641444     83.340859   13.893218 -0.405683  1.690065
[-0.4, -0.3)  425.941704     84.217430   25.579754  0.184349  0.836336
[-0.3, -0.2)  375.066116     82.471825   20.682568 -0.608185  1.064126
[-0.2, -0.1)  382.807263     82.146862   21.715927 -1.597008  1.126224
[-0.1, 0.0)   286.163241     87.789195   10.625407  0.713485  1.790242
[0.0, 0.1)    390.969763     83.196893   22.083714  0.068378  1.082724
[0.1, 0.2)    307.430278     84.485770   13.241163  0.444630  1.355545
[0.2, 0.3)    366.041800     81.623699   20.110776  0.085841  1.464471
[0.3, 0.4)    342.713428     74.441222   21.195090 -0.193052  0.993808
[0.4, 0.5)    444.800092     77.272327   33.134601  0.011396  0.839200

Books of histograms

A histogram Book acts like a Python dictionary, mapping string names to Hist objects. It provides the convenience of having only one object to fill (important in a complicated parallelization scheme), but also optimizes the calculation of those histograms to avoid unnecessary passes over the data.

>>> book = Book()
>>> for w in 0.1, 0.5, 0.9:
...     book["w %g" % w] = Hist(bin("w*left + (1-w)*right", 100, -5, 5), defs={"w": w})

>>> left = numpy.random.normal(-1, 1, 1000000)
>>> right = numpy.random.normal(1, 1, 1000000)
>>> book.fill(left=left, right=right)            # one "fill" for all histograms

>>> overlay(book["w 0.1"].step(),
...         book["w 0.5"].step(),
...         book["w 0.9"].step()).to(canvas)

docs/source/intro-13.png

In the above, we created three similar histograms, differing only in how to weight two subexpressions. The use of defs for substituting constants (or any expression) makes it easier to generate many histograms in a loop.

Note that the number of bins (memory use) scales as

(B 1 × ... × B n × (P 1 + ... + P m)) 1 + ... + (B 1 × ... × B n × (P 1 + ... + P m)) k

where B i is the number of bins in non-profile axis i, P i is the number of bins in profile axis i, and the whole expression is repeated for each histogram k in a book. That is, books add memory use, non-profile axes multiply, and profile axes add within the non-profile axes.

Manipulation methods

select

Select a set of bins with a boolean expr, returning a new Hist. Cut boundaries may be approximate (within tolerance), but the inequalities must be exact.

For example, if the low edge of each bin is closed, attempting to cut above it without including it is an error, as is attempting to cut below it with including it:

>>> h = Hist(bin("x", 100, -5, 5, closedlow=True))
>>> h.select("x <= 0")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "histbook/proj.py", line 230, in select
    return self._select(expr, tolerance)
  File "histbook/proj.py", line 328, in _select
    raise ValueError("no axis can select {0} (axis {1} has the wrong inequality; low edges are {2})"
                     .format(repr(str(expr)), wrongcmpaxis, "closed" if wrongcmpaxis.closedlow else
                     "open"))
ValueError: no axis can select 'x <= 0' (axis bin('x', 100, -5.0, 5.0) has the wrong inequality;
            low edges are closed)

whereas

>>> h.select("x < 0")
Hist(bin('x', 50, -5.0, 0.0, overflow=False, nanflow=False))

Any selection other than "x == nan" eliminates the nanflow because every comparison with "not a number" should yield False. (So technically, "x == nan" shouldn't work— this deviation from strict IEEE behavior is for convenience.)

Selections can never select a partial bin, so filling a histogram and then selecting from it should yield exactly the same result as filtering the data before filling.

Categorical groupby axes can be selected with Python's in operator and constant sets (necessary because there are no comparators for categorical data other than ==, !=, and in).

>>> h = Hist(groupby("c"))
>>> h.fill(c=["one", "two", "two", "three", "three", "three"])
>>> h.pandas()
       count()  err(count())
c
one        1.0      1.000000
three      3.0      1.732051
two        2.0      1.414214
>>> h.select("c in {'one', 'two'}").pandas()
     count()  err(count())
c
one      1.0      1.000000
two      2.0      1.414214

project

Reduces the number of non-profile axes to the provided set, *axis, by summing over all other non-profile axes.

All internal data are sums that are properly combined by summing. For instance, histograms are represented by a count (unweighted) or a sum of weights and squared-weights (weighted), and profiles are represented by a sum of the quantity times weight and a sum of the squared-quantity times weight.

drop

Eliminates all profile axes except the provided set, *profile.

If a Hist were represented as a table, non-profile axes form a compound key but profile axes are simple columns, which may be dropped without affecting any other data.

rebin, rebinby

Eliminates or sums neighboring bins to reduce the number of bins in an axis to edges or by a multiplicative factor.

A Hist with detailed binning in two dimensions can be plotted against one axis with rebinned overlays in the other axis and vice-versa.

Combining histograms

Separately filled histograms (Hist or Book) that represent the same data can be combined by adding them with the + operator. This simply adds all bins (like ROOT's hadd).

However, you may also want to combine qualitatively different data while maintaining their distinction as a new categorical axis. A common reason for this is to make a stacked plot of different distributions, such as different Monte Carlo samples in physics. For this, you use the Hist.group or Book.group static methods.

For example, suppose that we have two histograms filled with different data:

>>> h1 = Hist(bin("x", 10, -5, 5))
>>> h2 = Hist(bin("x", 10, -5, 5))
>>> h1.fill(x=numpy.random.normal(-2.5, 1, 1000000))
>>> h2.fill(x=numpy.random.normal(2.5, 1, 1000000))

Adding them mixes data into the same bins, after which they are no longer seperable.

>>> (h1 + h2).pandas()
               count()  err(count())
x
[-inf, -5.0)    6228.0     78.917679
[-5.0, -4.0)   60582.0    246.134110
[-4.0, -3.0)  241904.0    491.837371
[-3.0, -2.0)  383531.0    619.298797
[-2.0, -1.0)  241015.0    490.932786
[-1.0, 0.0)    66541.0    257.955423
[0.0, 1.0)     66982.0    258.808810
[1.0, 2.0)    240963.0    490.879822
[2.0, 3.0)    383046.0    618.907101
[3.0, 4.0)    242198.0    492.136160
[4.0, 5.0)     60726.0    246.426460
[5.0, inf)      6284.0     79.271685
{NaN}              0.0      0.000000

But grouping them creates a new categorical axis, "source" by default, where each distribution is associated with an assigned categorical value.

>>> h = Hist.group(a=h1, b=h2)
>>> h.pandas()
                      count()  err(count())
source x
a      [-inf, -5.0)    6228.0     78.917679
       [-5.0, -4.0)   60582.0    246.134110
       [-4.0, -3.0)  241904.0    491.837371
       [-3.0, -2.0)  383528.0    619.296375
       [-2.0, -1.0)  240761.0    490.674026
       [-1.0, 0.0)    60570.0    246.109732
       [0.0, 1.0)      6187.0     78.657485
       [1.0, 2.0)       236.0     15.362291
       [2.0, 3.0)         4.0      2.000000
       [3.0, 4.0)         0.0      0.000000
       [4.0, 5.0)         0.0      0.000000
       [5.0, inf)         0.0      0.000000
       {NaN}              0.0      0.000000
b      [-inf, -5.0)       0.0      0.000000
       [-5.0, -4.0)       0.0      0.000000
       [-4.0, -3.0)       0.0      0.000000
       [-3.0, -2.0)       3.0      1.732051
       [-2.0, -1.0)     254.0     15.937377
       [-1.0, 0.0)     5971.0     77.272246
       [0.0, 1.0)     60795.0    246.566421
       [1.0, 2.0)    240727.0    490.639379
       [2.0, 3.0)    383042.0    618.903870
       [3.0, 4.0)    242198.0    492.136160
       [4.0, 5.0)     60726.0    246.426460
       [5.0, inf)      6284.0     79.271685
       {NaN}              0.0      0.000000
>>> beside(h.area("x"), h.stack("source").area("x")).to(canvas)

docs/source/intro-14.png

For both types of combination, all axes of the Hist or all histograms in the Book must be identical.

Tabular output

table

Presents data from the histogram as a Numpy array,

fraction

Presents cut fractions (cut efficiencies) as a function of non-profile axes for each cut.

pandas

Presents a Hist.table as a Pandas DataFrame if all *axis are profiles or Hist.fraction if all *axis are cuts.

Plotting methods

An n-dimensional histogram is plotted by spreading its bins across the horizontal axis, across overlaid curves, across a cumulative stack, or across horizontal or vertical side-by-side plots. Any dimensions not spread across a graphical channel are summed, so these plots are a kind of projection. A typical use is to select and rebin first, spread zero or more axes across overlays or trellis (side-by-side) channels, then spread the last axis across horizontal bins.

The syntax for these operations is fluent: histogram-dot-operation-dot-operation-dot-plot. A chain of selection/rebinning/plotting operations ends with .vegalite() (for a Vega-Lite JSON object) or .to(canvas) (where canvas is a callable that draws the Vega-Lite). Chainable plotting operations are:

  • overlay(axis) to spread the bins of axis across overlaid curves
  • stack(axis, order=None) to stack them cumulatively with an optional order (can only be used if area is the terminal operation in the chain)
  • beside(axis) to spread the bins of axis across horizontally arranged plots
  • below(axis) to spread the bins of axis across vertically arranged plots

The following plotting operations are terminal: they must be last in a chain.

In addition, terminated plotting chains can be combined with the following operations. The output of these functions can be plotted with .vegalite() (for a Vega-Lite JSON object) or .to(canvas).

Exporting to ROOT

Hist.root(*axis)

Returns a PyROOT histogram projected on *axis. The type (TH1, TH2, TProfile, ...) depends on the Hist.

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