[matplotlib-devel] plot() with a nearly-constant sequence doesn't always work

[Dan Drake]

Hello matplotlib developers,

In Sage, we've run into a problem with plotting a sequence whose
y-values change by very small amounts. Here's an example that doesn't
use anything from Sage:

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()

We're using matplotlib 1.1. Here's a trac ticket where we are working on
this: http://trac.sagemath.org/sage_trac/ticket/11973. One of our
developers suspects matplotlib.ticker.MaxNLocator.bin_boundaries but we
don't really know.

Thanks for any help or comments!

Dan

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---  Dan Drake
-  http://mathsci.kaist.ac.kr/~drake
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Re: [matplotlib-devel] plot() with a nearly-constant sequence doesn't always work

[Dan Drake]

On Sat, 26 May 2012 at 03:30PM -1000, Eric Firing wrote:
> It is easy enough to remove the immediate roadblock in scale_range,
> but that just opens up a can of floating point worms.  The axis spines
> start getting misplaced, for example, as the range being plotted gets
> too small relative to the offset.  Straightening all this out, or even
> substantially improving it, is potentially tricky. To the extent that
> it can be done, it will have to be in master, which already includes
> one cleanup of a floating point kluge.
> 
> Note that part of the problem here is that in your example we are
> running out of precision.  The best way to handle it is to subtract an
> offset first, and just plot the deviation from that offset.  I think
> this is best done at the application level.  We can probably make
> mpl's handling of the problem degrade more gracefully, however, than
> it does at present.

Thanks for your help. I'll look at your links and see what we can do.

Dan

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