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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: QuadMesh.get_cursor_data() returns incorrect value","articleBody":"### Bug summary\n\n`QuadMesh.get_cursor_data()` returns the incorrect value, but only for some figure sizes, when an event is near the boundary between two cells and the axis is categorical. By 'incorrect', I mean it doesn't align with the event's `xdata` and `ydata` values.\r\n\r\nIn the below sample I get the cell value by two means and compare them:\r\n* by rounding the `xdata` and `ydata` values from the event to index into the underlying data\r\n* using `get_cursor_data`.\r\n\r\nThey _almost_ always match, but not always.\n\n### Code for reproduction\n\n```python\nimport matplotlib.pyplot as plt\r\n\r\nimport numpy as np\r\n\r\nplt.rcParams[\"backend\"] = \"TKAgg\"\r\nplt.ion()\r\nfig, ax = plt.subplots()\r\n\r\ndata = np.arange(6).reshape(2, 3)\r\n\r\nqm = ax.pcolormesh(\r\n    [\"col 1\", \"col 2\", \"col 3\"],\r\n    [\"row 1\", \"row 2\"],\r\n    data,\r\n)\r\n\r\n\r\ndef on_mouse_move(event):\r\n    if not event.inaxes:\r\n        return\r\n    cd = qm.get_cursor_data(event)\r\n    if cd is None:\r\n        return\r\n\r\n    # The value of the cell, according to get_cursor_data()\r\n    cd_val = cd[0]\r\n\r\n    # The value of the cell, according to the data row/col\r\n    data_val = data[round(event.ydata), round(event.xdata)]\r\n\r\n    if cd_val == data_val:\r\n        print(\"Fine\")\r\n    else:\r\n        print(event)\r\n        print(cd)\r\n        raise ValueError(f\"Value mismatch. {cd_val} != {data_val}\")\r\n\r\n\r\nfig.canvas.mpl_connect(\"motion_notify_event\", on_mouse_move)\r\nfig.canvas.manager.window.geometry(\"800x1054+100+100\")\n```\n\n\n### Actual outcome\n\nAs you hover the cursor right in the middle where two cells touch, it raises the error when the values don't match.\r\n![image](https://github.com/matplotlib/matplotlib/assets/4443482/b81716a5-5866-4fcc-b036-31bb654a4ca2)\r\n\n\n### Expected outcome\n\nThe two values should always match.\n\n### Additional information\n\nTrying to track down the issue, I noticed that it only happens at some figure heights. When I changed from my default to pretty much anything else, it was difficult to get the error.\r\n\r\nSo I can confirm that it isn't _caused_ by the call to `fig.canvas.manager.window.geometry`, but if you don't do this you likely won't be able to replicate the issue.\r\n\r\nThe real world situation where I discovered this: I was producing a tooltip for a heatmap. I got the y label (Country) using `ax.format_ydata(event.ydata)` and the cell value using `quad_mesh.get_cursor_data(event)`. I only noticed when the cells are quite  small, it gives me incorrect information a lot, e.g. Canada has no bananas, so should show `nan` (short for no nanas), but it shows the value from Cape Verde instead.\r\n\r\n![image](https://github.com/matplotlib/matplotlib/assets/4443482/1472ec16-ca54-4873-a7af-1736c35971df)\r\n\r\nZoomed in\r\n![image](https://github.com/matplotlib/matplotlib/assets/4443482/b44602c0-9acc-4efe-856e-39e639b42525)\r\n\r\nYou can also replicate this without interaction by just firing a bunch of mouse events near the border, this will fail regardless of the figure size.\r\n```py\r\nfig.canvas.draw()\r\n\r\nfor ax_y in np.linspace(0.49, 0.51, 100):\r\n    x, y = ax.transAxes.transform((0.5, ax_y))\r\n    mouse_event = MouseEvent(\r\n        name=\"motion_notify_event\",\r\n        canvas=fig.canvas,\r\n        x=x,\r\n        y=y,\r\n    )\r\n    on_mouse_move(mouse_event)\r\n```\r\n\r\nI'm not sure what the fix would look like, it seems to go quite deep. A good interim step would be a note in the docs for `get_cursor_data()` warning that it isn't guaranteed to align with `xdata` and `ydata` near the borders.\n\n### Operating system\n\nWindows\n\n### Matplotlib Version\n\n3.8.0\n\n### Matplotlib Backend\n\nTKAgg\n\n### Python version\n\n3.10\n\n### Jupyter version\n\n_No response_\n\n### Installation\n\npip","author":{"url":"https://github.com/davidgilbertson","@type":"Person","name":"davidgilbertson"},"datePublished":"2023-12-26T20:55:28.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/27570/matplotlib/issues/27570"}