demo.md

A Brief Demonstration of Gadfly

Gadfly is an implementation of a "grammar of graphics" style statistical graphics system for Julia. Though currently far from complete, some basics are up and running. This document will show some examples of what currently works while giving you a feel for the overall system.

To begin, we need some data. The data Gadfly plots is always given in the form of a DataFrame. We'll pick and choose some examples from the RDatasets packages.

Fisher's iris data set is a good starting point.

load("Gadfly")
using Gadfly

load("RDatasets")
using RDatasets

iris = data("datasets", "iris")

The plot function in Gadfly is of the form:

plot(data::DataFrame, mapping::Dict, elements::Element...)

The first argument is the data to be plotted, the second is a dictionary mapping "aesthetics" to columns in the data frame, and this is followed by some number of elements, which are the nouns and verbs, so to speak, that form the grammar.

Let's get to it.

p = plot(iris, {:x => "Sepal.Length", :y => "Sepal.Width"}, Geom.point)

This produces a Plot object. We can turn it into a graphic by calling render on it, and this can then in turn be drawn on one or more backends.

g = render(p)

img = SVG("iris_plot.svg", 6inch, 4inch)
draw(img, g)
finish(img)

Now we have the following charming little SVG image.

For the rest of the demonstrations, we'll omit the render and draw calls for brevity.

In this plot we've mapped the x aesthetic to the Sepal.Length column and the y aesthetic to the Sepal.Width. The last argument, Geom.point, is a geometry element which takes bound aesthetics and render delightful figures. Adding other geometries produces layers, which may or may not result in a coherent plot.

p = plot(iris, {:x => "Sepal.Length", :y => "Sepal.Width"},
         Geom.point, Geom.line)

This is the grammar of graphics equivalent of "colorless green ideas sleep furiously". It is valid grammar, but not particularly meaningful.

Color

Let's do add something meaningful by mapping the color aesthetic.

p = plot(iris, {:x => "Sepal.Length", :y => "Sepal.Width", :color => "Species"},
         Geom.point)

Ah, a scientific discovery: Setosa has short but wide sepals!

Color scales in Gadfly by default are produced from perceptually uniform colorspaces (LUV/LCHuv or LAB/LCHab), though it supports RGB, HSV, HLS, XYZ, and converts arbitrarily between these. Of course, CSS/X11 named colors work too: "old lace", anyone?

Scale transforms

Scale transforms also work as expected. Let's look at some data where this is useful.

mammals = data("MASS", "mammals")
p = plot(mammals, {:x => "body", :y => "brain"}, Geom.point)

This is no good, the whales are ruining things for us. Putting both axis on a log-scale clears things up.

p = plot(mammals, {:x => "body", :y => "brain"},
         Geom.point, Scale.x_log10, Scale.y_log10)

Discrete scales

Since all continuous analysis is just degenerate discrete analysis, let's take a crack at the latter using some fuel efficiency data.

gasoline = data("Ecdat", "Gasoline")

p = plot(gasoline, {:x => "year", :y => "lgaspcar", :color => "country"},
         Geom.point, Geom.line)

# Make this image wider so the axis labels fit
g = render(p)
img = SVG("gasoline_plot.svg", 9inch, 4inch)
draw(img, g)
finish(img)

We could have added Scale.x_discrete explicitly, but this is detected and the right default is chosen. This is the case with most of elements in the grammar: we've omitted Scale.x_continuous and Scale.y_continuous in the previous plots, as well as Coord.cartesian, and guide elements such as Guide.color_key, Guide.x_ticks, Guide.XLabel, and so on. As much as possible the system tries to fill in the gaps with reasonable defaults.

Rendering

Gadfly uses a custom graphics library called Compose, which is an attempt at a more elegant, purely functional take on the R grid package. It allows mixing of absolute and relative units and complex coordinate transforms. The primary backend is a native SVG generator (almost native: it uses pango to precompute text extents), though there is also a (currently, slightly buggy) Cairo backend.

Building graphics declaratively let's you do some fun things. Like stick to plots together:

fig1a = render(plot(iris, {:x => "Sepal.Length", :y => "Sepal.Width"},
                    Geom.point))
fig1b = render(plot(iris, {:x => "Sepal.Width"}, Geom.bar))
fig1 = hstack(fig1a, fig1b)

img = SVG("fig1.svg", 9inch, 4inch)
draw(img, fig1)
finish(img)

Ultimately this will make more complex visualizations easier to build. For example, facets, plots within plots, and so on.

Interactivity

One advantage of generating our own SVG is that the files are much more compact than those produced by Cairo, by virtue of having a higher level API. Another advantage is that we can annotate our SVG output and embed javascript code to provide some level of dynamism.

As a proof of concept, color keys support toggling categories on and off by clicking the category name. Javascript is disabled in GitHub rendered markdown files, but works by viewing one the figures directly.

Though not a replacement for full-fledged custom interactive visualizations of the sort produced by d3, this sort of mild interactivity can improve a lot of standard plots. The fuel efficiency plot is made more clear by toggling off some of the countries, for example.