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fill_between incorrect with log y-axis and value 0 #8623

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cdeil opened this issue May 15, 2017 · 11 comments
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fill_between incorrect with log y-axis and value 0 #8623

cdeil opened this issue May 15, 2017 · 11 comments
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status: confirmed bug
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v2.1.1

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@cdeil
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cdeil commented May 15, 2017

fill_between doesn't give the result I expect if using a log y-axis and having a value of 0 in the y1 array.

Example:

%matplotlib inline
import numpy as np
import matplotlib.pyplot as plt

x = np.array([1, 2, 3, 4, 5])
y1 = np.array([9, 2, 8, 3, 0])
y2 = np.array([10, 10, 10, 10, 10])

plt.plot(x, y1, lw=3, color='green')
plt.plot(x, y2, lw=3, color='red')
plt.fill_between(x, y1, y2)
plt.loglog()

Actual outcome:

The polygon created by fill_between somehow connects the last and first point, resulting in a completely incorrect patch:

image

Expected outcome

I was expecting / hoping that the band would be filled correctly and simply clipped (i.e. the result I get when changing the last y1 value from 0 to e.g. 1e-10). In the plot, the blue fill should be between the red and green line.

Matplotlib version

This is with Matplotlib 2.0.0.
(on Macports, but I don't think that matters)
@cdeil cdeil mentioned this issue May 15, 2017
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@dstansby dstansby added this to the 2.1 (next point release) milestone May 15, 2017
@dstansby
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Confirmed on master, thanks for the code sample.

@dstansby
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Slightly abstractly, y=0 is mapped to x = -infinity on a log scale, so I would expect the fill between to be clipped at the vertical line x=4 in the above example, and for the x=5, y=0 point to be ignored.

@cdeil
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cdeil commented May 15, 2017

@dstansby - Thanks for looking into the issue!

When creating the issue I forgot to mention #7371 .
I don't quite follow the discussion there, but it might be relevant to decide what to do here.
For me, any solution that doesn't have this issue that the last y1 point connects to the first y2 point would be fine.

Also, if someone else has this issue, I wanted to mention that it's possible to work around the issue using where and filtering out the non-positive y values:

where = (y_hi > 0) & (y_lo > 0) & (other possible selections you might have)
ax.fill_between(energy.value, y_lo.value, y_hi.value, where=where)

This is what I'm doing for now.

@ghost
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ghost commented May 30, 2017 •
edited by ghost
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@cdeil and @dstansby I think this issue was already dealt with using the current latest code in master. You just have to call plt.loglog(nonposx='clip', nonposy='clip') (rather than the default plt.loglog() with no **kwargs). At least, it works on my machine. I didn't try it out, but I don't think you need nonposx = 'clip' if there's no 0 in your x-axis.
figure_1

@cdeil
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cdeil commented Jul 3, 2017

@Tuan333 - Yes, indeed nonposy='clip' resolves the issue, thanks for pointing it out.

The main remaining issue is that we have to teach our users about it, because if they call plt.loglog without that option after calling the plotting functions from our package, they will still see that issue.

Close this issue?
Or is there anything that can be improved on the matplotlib side (e.g. change default behaviour)?

@adonath adonath mentioned this issue Jul 3, 2017
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@anntzer
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anntzer commented Jul 3, 2017

Probably related to #8537?

@dstansby
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dstansby commented Jul 3, 2017 via email

@dstansby dstansby mentioned this issue Jul 3, 2017
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@tacaswell
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@dstansby Won't #8836 make this worse (by changing more things to mask rather than clip)?

@dstansby
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dstansby commented Jul 6, 2017

I don't think it'll make it worse than the image in the original bug report, but it certainly doesn't fix it.

@tacaswell tacaswell mentioned this issue Jul 11, 2017
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@tacaswell tacaswell modified the milestones: 2.1 (next point release), 2.2 (next next feature release) Sep 24, 2017
@hsandmeyer hsandmeyer mentioned this issue Oct 17, 2017
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@afvincent
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Currently, the MWE results in
mwe_with_master
with Matplotlib from master (2.1.0.post476+g30b58001f). (The issue is still present with Matplotlib 2.1 from conda)

@tacaswell @anntzer
Should we close this issue (didn't #9477 fixed it?)?

@tacaswell
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I think this is fixed by #9477

@tacaswell tacaswell modified the milestones: v2.2, v2.1.1 Nov 1, 2017
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@tacaswell @cdeil @anntzer @dstansby @afvincent