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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: Secondary axis does not support Transform as functions","articleBody":"### Bug summary\n\nThe [documentation for `secondary_[xy]axis`](https://matplotlib.org/stable/api/_as_gen/matplotlib.axes.Axes.secondary_xaxis.html#matplotlib.axes.Axes.secondary_xaxis) claims to accept a `Transform` for the `functions` parameter.\r\n\r\nHowever, the checks in `set_functions` do not accept `Transform`:\r\nhttps://github.com/matplotlib/matplotlib/blob/3f4d4c10d40c1c60e9572b5e4702a463e2a06747/lib/matplotlib/axes/_secondary_axes.py#L147-L158\r\n\r\nand any usage of the internal attribute seems to assume a tuple of callables:\r\nhttps://github.com/matplotlib/matplotlib/blob/3f4d4c10d40c1c60e9572b5e4702a463e2a06747/lib/matplotlib/axes/_secondary_axes.py#L193-L194\r\nhttps://github.com/matplotlib/matplotlib/blob/3f4d4c10d40c1c60e9572b5e4702a463e2a06747/lib/matplotlib/axes/_secondary_axes.py#L216\r\n\r\nBoth this documentation and the checks have been in the code since it was added in #11859.\n\n### Code for reproduction\n\n```python\nimport matplotlib.pyplot as plt\r\nfrom matplotlib.transforms import IdentityTransform\r\n\r\nfig, ax = plt.subplots()\r\nsecax = ax.secondary_xaxis('top', functions=IdentityTransform)\r\n# OR:\r\nsecax = ax.secondary_xaxis('top', functions=IdentityTransform())\r\n# OR:\r\nsecax = ax.secondary_xaxis('top', functions=(IdentityTransform, IdentityTransform))\r\n# OR:\r\nsecax = ax.secondary_xaxis('top', functions=(IdentityTransform(), IdentityTransform()))\n```\n\n\n### Actual outcome\n\nWith a single transform (class or object), _or_ a tuple of transform objects, it fails in the setter:\r\n```pytb\r\nTraceback (most recent call last):\r\n  File \"galleries/examples/subplots_axes_and_figures/secondary_axis.py\", line 40, in \u003cmodule\u003e\r\n    secax = ax.secondary_xaxis('top', functions=IdentityTransform)\r\n  File \"lib/matplotlib/axes/_axes.py\", line 586, in secondary_xaxis\r\n    secondary_ax = SecondaryAxis(self, 'x', location, functions,\r\n  File \"lib/matplotlib/axes/_secondary_axes.py\", line 42, in __init__\r\n    self.set_functions(functions)\r\n  File \"lib/matplotlib/axes/_secondary_axes.py\", line 155, in set_functions\r\n    raise ValueError('functions argument of secondary axes '\r\nValueError: functions argument of secondary axes must be a two-tuple of callable functions with the first function being the transform and the second being the inverse\r\n```\r\nWith a tuple of transform classes, the setter allows it, but it fails when drawing:\r\n```\r\nTraceback (most recent call last):\r\n  File \"lib/matplotlib/backends/backend_qt.py\", line 461, in _draw_idle\r\n    self.draw()\r\n  File \"lib/matplotlib/backends/backend_agg.py\", line 401, in draw\r\n    self.figure.draw(self.renderer)\r\n  File \"lib/matplotlib/artist.py\", line 95, in draw_wrapper\r\n    result = draw(artist, renderer, *args, **kwargs)\r\n  File \"lib/matplotlib/artist.py\", line 72, in draw_wrapper\r\n    return draw(artist, renderer)\r\n  File \"lib/matplotlib/figure.py\", line 3115, in draw\r\n    artists = self._get_draw_artists(renderer)\r\n  File \"lib/matplotlib/figure.py\", line 225, in _get_draw_artists\r\n    child.apply_aspect(\r\n  File \"lib/matplotlib/axes/_secondary_axes.py\", line 120, in apply_aspect\r\n    self._set_lims()\r\n  File \"lib/matplotlib/axes/_secondary_axes.py\", line 216, in _set_lims\r\n    lims = self._functions[0](np.array(lims))\r\n  File \"lib/matplotlib/transforms.py\", line 1759, in __init__\r\n    super().__init__(*args, **kwargs)\r\n  File \"lib/matplotlib/transforms.py\", line 125, in __init__\r\n    self._shorthand_name = shorthand_name or ''\r\nValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()\r\n```\n\n### Expected outcome\n\nAt least one of the options works, or it should not be documented as accepted.\n\n### Additional information\n\nI noticed this oddity in the types while reviewing #25224, but not sure what we intend here.\n\n### Operating system\n\n_No response_\n\n### Matplotlib Version\n\n3f4d4c10d40c1c60e9572b5e4702a463e2a06747\n\n### Matplotlib Backend\n\n_No response_\n\n### Python version\n\n_No response_\n\n### Jupyter version\n\n_No response_\n\n### Installation\n\ngit checkout","author":{"url":"https://github.com/QuLogic","@type":"Person","name":"QuLogic"},"datePublished":"2023-04-15T06:07:32.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/25691/matplotlib/issues/25691"}