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Fix specifying number of levels with log contour#27576

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dstansby merged 8 commits intomatplotlib:mainfrom

dstansby:log-contour-levels

Apr 30, 2025

Merged

Fix specifying number of levels with log contour#27576
dstansby merged 8 commits intomatplotlib:mainfrom
dstansby:log-contour-levels

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dstansby commentedDec 28, 2023•
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PR summary

When plotting contours with a log norm, passing an integer value to thelevels argument to cap the maximum number of contour levels now works as intended. Partially addresses#19856 and replaces#25149.

I've maually checked that the added test fails before this fix. Testing with the following script:

importmatplotlib.pyplotaspltfrommatplotlib.colorsimportLogNormimportnumpyasnpx,y=np.mgrid[1:10:0.1,1:10:0.1]data=np.abs(np.sin(x)*np.exp(y))forn_levelsinrange(2,20):fig,ax=plt.subplots()im=ax.contourf(x,y,data,norm=LogNorm(),levels=n_levels)cbar=fig.colorbar(im,ax=ax)n_ticks=len(cbar.ax.get_yticklabels())print(f"Requested max{n_levels+1}, got{n_ticks-1}")

Gives on currentmain:

Requested max 3, got 7Requested max 4, got 7Requested max 5, got 7Requested max 6, got 7Requested max 7, got 7Requested max 8, got 7Requested max 9, got 7Requested max 10, got 7Requested max 11, got 7Requested max 12, got 7Requested max 13, got 7Requested max 14, got 7Requested max 15, got 7Requested max 16, got 7Requested max 17, got 7Requested max 18, got 7Requested max 19, got 7Requested max 20, got 7

and with this PR:

Requested max 3, got 3Requested max 4, got 4Requested max 5, got 4Requested max 6, got 4Requested max 7, got 7Requested max 8, got 7Requested max 9, got 7Requested max 10, got 7Requested max 11, got 7Requested max 12, got 7Requested max 13, got 7Requested max 14, got 7Requested max 15, got 7Requested max 16, got 7Requested max 17, got 7Requested max 18, got 7Requested max 19, got 7Requested max 20, got 7

PR checklist

"closes #0000" is in the body of the PR description tolink the related issue
new and changed code istested
[n/a]Plotting related features are demonstrated in anexample
New Features andAPI Changes are noted with adirective and release note
Documentation complies withgeneral anddocstring guidelines

dstansby mentioned this pull request

Dec 28, 2023

Addition of numticks=N in LogLocator#20950

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dstansby added the topic: contour label

Dec 28, 2023

dstansby marked this pull request as ready for review

December 28, 2023 21:13

ksunden reviewed

Jan 9, 2024

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January 10, 2024 10:48

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Jan 19, 2024

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February 17, 2024 10:07

github-actionsbot added the status: needs rebase label

Sep 7, 2024

ebarcelosf mentioned this pull request

Nov 13, 2024

Uptade fix number of levels with log contour#29137

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dstansby force-pushed thelog-contour-levels branch fromf495d8d to9cd6171Compare

March 24, 2025 21:49

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March 27, 2025 09:31

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timhoffm reviewed

Apr 8, 2025

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dstansby commentedApr 9, 2025

Thanks - as well as applying your suggestions, I also factored out the logic to set the locator if it's not already set. This allows the long docstrings to be split up, and makes it explicit in_process_contour_level_args where the default locator is being set, instead of hiding it in_autolev.

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Apr 9, 2025

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lib/matplotlib/contour.py

Comment on lines 1028 to 1029

		if args:
		# Set if levels manually provided
		levels_arg = args[0]

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timhoffmApr 9, 2025•
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This could still be made clearer / the current code is still convoluted (but possibly better as a followup PR to not make too many changes and complicate review)

args is not the originalargs but stripped from leading[X, Y], Z. Effectively it can only be empty or contain a single element that containslevels passed positionally (which is supported, but a bit cumbersome to write out as a signature because the optional[X, Y] require to absorb all positional parameters in*args and we therefore have to reunite a possible positional level from args with the possible kwarg level.

The current implementation also has the surprising side effect that you can docontour(Z, levels1, levels=levels2) and the positionallevels1 is sliently ignored.

The only use ofargs here is to pass on a possible positionallevel. We can push the logic out of the function and unite the positional and kwarg paths oflevels in the caller_contour_args - where it belongs logically.

I think eventually we should also makelevels kw-only.contour(Z, 5) orcontour(X, Y, Z, [-1, 0, 1]) are not very readable anyway.