René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"contour edge case with all data below levels and a surrounding field of zeros","articleBody":"\u003c!--To help us understand and resolve your issue, please fill out the form to the best of your ability.--\u003e\r\n\u003c!--You can feel free to delete the sections that do not apply.--\u003e\r\n\r\n### Bug report\r\n\r\n**Bug summary**\r\n\r\n\u003c!--A short 1-2 sentences that succinctly describes the bug--\u003e\r\nIf I attempt to make a contour plot with data that is entirely below the lowest level, and the data has a field of zeros around it, I get an unexpected contour between the zeros and anything positive.\r\n\r\n**Code for reproduction**\r\n\r\n\u003c!--A minimum code snippet required to reproduce the bug.\r\nPlease make sure to minimize the number of dependencies required, and provide\r\nany necessary plotted data.\r\nAvoid using threads, as Matplotlib is (explicitly) not thread-safe.--\u003e\r\n\r\n```python\r\nimport numpy as np\r\nimport matplotlib.pyplot as plt\r\n\r\ny = np.linspace(-10, 10, 20)\r\nx_2d, y_2d = np.meshgrid(y, y)\r\n\r\ndata = np.cos(x_2d * np.pi/10.) + np.cos(y_2d * np.pi/10.)\r\nnp.place(data, data \u003c 0., 0.)\r\n\r\nlevels = [3, 6]\r\ncolors = ['b', 'r']\r\n\r\nplt.contour(data, levels=levels, colors=colors)\r\n\r\nplt.show()\r\n```\r\n\r\n**Actual outcome**\r\n\r\n\u003c!--The output produced by the above code, which may be a screenshot, console output, etc.--\u003e\r\n![bug_zeros_line](https://user-images.githubusercontent.com/10599679/117796800-4f6e8f80-b247-11eb-8d6c-d97c55057ea4.png)\r\n\r\n**Expected outcome**\r\n\r\nThe plot should be blank, which is what happens if the call to `np.place` is skipped.\r\n\r\n\u003c!--A description of the expected outcome from the code snippet--\u003e\r\n\u003c!--If this used to work in an earlier version of Matplotlib, please note the version it used to work on--\u003e\r\n\r\n**Matplotlib version**\r\n\u003c!--Please specify your platform and versions of the relevant libraries you are using:--\u003e\r\n  * Operating system: RHEL7.9\r\n  * Matplotlib version (`import matplotlib; print(matplotlib.__version__)`):  3.4.1\r\n  * Matplotlib backend (`print(matplotlib.get_backend())`): Qt5Agg\r\n  * Python version: 3.8.8\r\n  * Jupyter version (if applicable): N/A\r\n  * Other libraries: numpy 1.20.2\r\n\r\n\u003c!--Please tell us how you installed matplotlib and python e.g., from source, pip, conda--\u003e\r\n\u003c!--If you installed from conda, please specify which channel you used if not the default--\u003e\r\n\r\n","author":{"url":"https://github.com/rcomer","@type":"Person","name":"rcomer"},"datePublished":"2021-05-11T10:06:27.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/20203/matplotlib/issues/20203"}