list_plot of a nearly constant sequence fails

[dandrake]

At https://groups.google.com/d/topic/sage-devel/gu0Al01k-3Y/discussion I noted that list_plot/matplotlib are confused when given a nearly-constant sequence:

sage: list_plot([[0, 0.7886751345948214], [0.01, 0.788675134595], [0.02, 0.788675134595], [0.03, 0.788675134595], [0.04, 0.788675134595], [0.05, 0.788675134595], [0.06, 0.788675134595], [0.07, 0.788675134595], [0.08, 0.788675134595], [0.09, 0.788675134595], [0.1, 0.788675134595]])

fails with

...
/opt/sage/local/lib/python2.6/site-packages/matplotlib/axis.pyc in get_minorticklabels(self)
   1076     def get_minorticklabels(self):
   1077         'Return a list of Text instances for the minor ticklabels'
-> 1078         ticks = self.get_minor_ticks()
   1079         labels1 = [tick.label1 for tick in ticks if tick.label1On]
   1080         labels2 = [tick.label2 for tick in ticks if tick.label2On]

/opt/sage/local/lib/python2.6/site-packages/matplotlib/axis.pyc in get_minor_ticks(self, numticks)
   1190         'get the minor tick instances; grow as necessary'
   1191         if numticks is None:
-> 1192             numticks = len(self.get_minor_locator()())
   1193
   1194         if len(self.minorTicks) < numticks:

/opt/sage/local/lib/python2.6/site-packages/matplotlib/ticker.pyc in __call__(self)
   1424
   1425         # don't create minor ticks on top of existing major ticks
-> 1426         diff = 0.5 * abs(locs[1] - locs[0])
   1427         locs = [l for l in locs if (np.abs(l - majorlocs) > diff).all()]
   1428

IndexError: index out of bounds

(with 4.7.1 on 64-bit Linux.)

Since the usual plot() function works with constant functions, there should be a way to get this working. (Or, we should decide that this is a matplotlib bug and properly report it.)

CC: @novoselt

Component: graphics

Keywords: list_plot, matplotlib, sd40.5

Issue created by migration from https://trac.sagemath.org/ticket/11973

[kcrisman]

comment:1

Here are a whole bunch of (closed) related tickets.

• plotting constant functions throws an error #2409 plotting constant functions
• make plot() and parametric_plot() use fast_float on their functions #3952 make plot use fast-float
• plotting constant functions throws an error #2604 dup of plotting constant functions throws an error #2409
• Axes computation for constant function causes division by zero #4384 constant function causes division by zero
• rework save/show in plot, use Matplotlib's axes code, upgrade matplotlib #5448 use mpl axes and other improvements
  I suspect the last one has some clues, but I can't remember off-hand which of these actually fixed that before. Note that the error message here is different, because it's now an mpl error.

[jasongrout]

comment:3

This happens with plot too:

plot(lambda x: x*1+(1-x)*(1+1e-13),(x,0,1))

[jasongrout]

comment:4

It appears that the major locator is giving back bad values (i.e., somehow it is giving back [0,1,2] as the major tickmarks, which is then trimmed down to just 1, giving the minor locator error since there is only one major tickmark).

I think the problem is somewhere in matplotlib.ticker.MaxNLocator.bin_boundaries function, but I'm not sure.

[jasongrout]

comment:5

My test cases are:

sage: list_plot([0,0+1e-14]) # works

sage: list_plot([1,1+1e-14]) # doesn't work

[jasongrout]

comment:6

I think the next step would be to try to replicate it using only matplotlib. If we can do that, we can move the bug report upstream.

[jdemeyer]

Milestone sage-4.7.3 deleted

[sagetrac-dsm]

comment:8

I think we can bump this up:

import pylab
pylab.plot([0, 1], [0, 1e-14])
pylab.savefig("works.png")
pylab.close()
pylab.plot([0, 1], [1, 1+1e-14])
pylab.savefig("fails.png")
pylab.close()

reproduces the problem in mpl 1.2.x.

[jasongrout]

comment:9

Great; if you can post to the matplotlib mailing list and post the thread URL here, that would be great.

[novoselt]

Changed keywords from list_plot, matplotlib to list_plot, matplotlib, sd40.5

[dandrake]

comment:11

I'll contact matplotlib-devel and see what they think of the problem.

[dandrake]

comment:12

Interesting: the example I gave in the description works in 5.1beta0 (64-bit Linux), but the y-axis doesn't have any labels. The plot is correct for the given data, though.

[fchapoton]

comment:17

everything works in 6.6.rc0