René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"BUG: Contours with LogNorm","articleBody":"### Bug report\r\n\r\n**Bug summary**\r\nContour plots don't interact nicely with `matplotlib.colors.LogNorm`.\r\nLikely related to #19748\r\n\r\n**Code for reproduction**\r\n\r\n```python\r\nimport matplotlib.pyplot as plt\r\nfrom matplotlib.colors import LogNorm\r\nimport numpy as np\r\n\r\n# BUG: setting levels as a scalar has no effect and no warning or error is printed\r\nfig, ax = plt.subplots()\r\nim = ax.contourf(x, y, data, norm=LogNorm(), levels=4)\r\nfig.colorbar(im, ax=ax)\r\nplt.show()\r\n```\r\n![mpl_logcontours_bug1](https://user-images.githubusercontent.com/14075922/113481209-7f808100-9498-11eb-9085-96500a8fff0c.png)\r\n\r\n\r\n```python\r\n# kinda buggy too: setting levels as an array does have some of the desired effects\r\n# but ticks and ticklabels are incorrect\r\nfig, ax = plt.subplots()\r\nlevels = np.logspace(np.log10(data.min()), np.log10(data.max()), 5)\r\nim = ax.contourf(x, y, data, norm=LogNorm(), levels=levels)\r\nfig.colorbar(im, ax=ax)\r\nplt.show()\r\n```\r\n\r\n![mpl_logcontours_bug2](https://user-images.githubusercontent.com/14075922/113481210-80191780-9498-11eb-8d4c-ae34824841f6.png)\r\n\r\n\r\n**Expected outcome**\r\n\r\nThe experience should be comparable with what happens when the norm isn't specified: the `levels` kwarg should be usable in both cases.\r\nI note that the output from the first snippet is consistent with the case where neither `levels` or `norm` are specified, and is correct in that case, so it's definetely not _completely broken_ and I hope the fix is somewhat straightforward.\r\n\r\nI'm happy to take a look at the source and see if I can come up with an easy patch, but I'm not sure when I'll have time to dive in there myself. Any piece advice from maintainers or contributors is more than welcome !\r\n\r\n**Matplotlib version**\r\n\r\n  * Operating system: MacOS\r\n  * Matplotlib version (`import matplotlib; print(matplotlib.__version__)`): 3.4.1\r\n  * Matplotlib backend (`print(matplotlib.get_backend())`): `module://ipykernel.pylab.backend_inline`\r\n  * Python version: 3.9.1\r\n  * Jupyter version (if applicable): NA\r\n  * Other libraries: \r\n\r\nI installed matplotlib using pip.","author":{"url":"https://github.com/neutrinoceros","@type":"Person","name":"neutrinoceros"},"datePublished":"2021-04-03T14:32:13.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":15},"url":"https://github.com/19856/matplotlib/issues/19856"}