René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Secondary axis called with [0, 1] might produce exceptions in case these are invalid data","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\nTransform functions for the new feature `secondary_xaxis`  are always called with 0 and 1 at\r\ninstantiation, which might be invalid input for transformations.\r\n\r\n**Code for reproduction**\r\n\r\nE.g. trying to get a secondary axis showing mjd for a date axis\r\n\r\n```python\r\nimport matplotlib.pyplot as plt\r\nfrom matplotlib.dates import date2num, num2date\r\nfrom datetime import datetime, timedelta\r\nimport numpy as np\r\n\r\n\r\nMJDREF = datetime(1858, 11, 17, 0)\r\n\r\n\r\ndef trans(mpl_date):\r\n    print(mpl_date)\r\n    return (num2date(mpl_date) - MJDREF).total_seconds() / 86400\r\n\r\n\r\ndef inv(mjd):\r\n    return date2num(MJDREF + timedelta(days=mjd))\r\n\r\n\r\nt = np.array(['2018-01-01', '2018-03-03', '2018-04-10'], dtype='datetime64[s]')\r\n\r\n\r\nfig, ax = plt.subplots()\r\nax.plot(t, [1, 2, 1])\r\n\r\nt_ax = ax.secondary_xaxis('top', functions=(trans, inv))\r\n```\r\n\r\nResults in:\r\n\r\n```\r\nValueError: Cannot convert 0 to a date.  This often happens if non-datetime values are passed to an axis that expects datetime objects.\r\n```\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, but should be indepenent\r\n  * Matplotlib version:  3.1\r\n  * Matplotlib backend: all\r\n  * Python version: 3.7\r\n\r\n\r\n","author":{"url":"https://github.com/maxnoe","@type":"Person","name":"maxnoe"},"datePublished":"2019-05-20T10:12:20.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":9},"url":"https://github.com/14270/matplotlib/issues/14270"}