René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"pcolormesh: shading='nearest' and non-monotonic coordinates","articleBody":"# Bug report\r\n\r\n**Bug summary**\r\n\r\n#16258 deprecates `shading=\"flat\"` when data and coordinates have the same shape and introduces `shading=\"nearest\"` which interpolates the coordinates. However, the logic to interpolate the coordinates breaks down if they are not monotonically increasing.\r\n\r\nhttps://github.com/matplotlib/matplotlib/blob/85117715a154d1027c6e204d2ee581fa787f423d/lib/matplotlib/axes/_axes.py#L5684\r\n\r\nOf course non-monotonic coordinates are a bit exotic but they make sense for geographic data.\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\n\r\ndef _interp_grid(X):\r\n    # helper for below\r\n    if np.shape(X)[1] \u003e 1:\r\n        dX = np.diff(X, axis=1)/2.\r\n        X = np.hstack((X[:, [0]] - dX[:, [0]],\r\n                       X[:, :-1] + dX,\r\n                       X[:, [-1]] + dX[:, [-1]]))\r\n    return X\r\n\r\nx = np.array([1, 2, 0])\r\ny = np.array([1, 2])\r\n\r\nX, Y = np.meshgrid(x, y)\r\n\r\n_interp_grid(X)\r\n```\r\n\r\n**Actual outcome**\r\n\r\nResults in\r\n```python\r\narray([[ 0.5,  1.5,  1. , -1. ],\r\n       [ 0.5,  1.5,  1. , -1. ]])\r\n```\r\n\r\nWhile having cooridinates `[1, 2, 0]` may seem just as wrong as coordinates `[0.5, 1.5, 1, -1]` the latter can cause some real-world problems in conjunction with cartopy (scitools/cartopy#1638):\r\n\r\n![shading_rasm_both](https://user-images.githubusercontent.com/10194086/90925277-3ab84600-e3f1-11ea-8aa5-8debfb465495.png)\r\n\r\n\r\n**Expected outcome**\r\n\r\n* A warning for non-monotonic data\r\n* A convenient option to restore the old behaviour, e.g. `shading=\"drop\"`\r\n\r\ncc @greglucas\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: Linux Mint\r\n  * Matplotlib version: '3.3.1'\r\n  * Matplotlib backend (`print(matplotlib.get_backend())`): TkAgg\r\n  * Python version: 3.8\r\n  * Jupyter version (if applicable):\r\n  * Other libraries: \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/mathause","@type":"Person","name":"mathause"},"datePublished":"2020-08-21T19:00:45.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":18},"url":"https://github.com/18317/matplotlib/issues/18317"}