René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[ENH]: minor ticks for symlog norms (with integer base)","articleBody":"### Problem\r\n\r\nAs of matplotlib 3.7.1,  `matplotlib.colors.SymLogNorm` doesn't produce any minor tick (that doesn't overlap with major ticks). I understand that default minor tick locations are absolutely not obvious in the general case and, where the log base can be any real number, however, I think there's a case to be had regarding the special (and very useful) case of *integer* log bases (10 and 2 are probably among the most useful bases all domains considered), in which the interval between two major ticks n and n+1 can be divided by the interval between ticks n-1 and n (where n+1 is farthest from 0).\r\n\r\nIf fact `matplotlib.colors.LogNorm` uses log10 and produces minorticks  by default. The difference in behaviour is perhaps best illustrated in a case where everything else is identical\r\n\r\n```python\r\nimport numpy as np\r\nimport matplotlib.pyplot as plt\r\nfrom matplotlib.colors import SymLogNorm, LogNorm\r\n\r\nprng = np.random.RandomState(0)\r\ndata = 50 * (prng.random_sample((10, 10)))\r\n\r\nfig, axs = plt.subplots(ncols=2, figsize=(10,3))\r\n\r\nfor ax, norm in zip(axs, [LogNorm(), SymLogNorm(base=10, linthresh=1)]):\r\n    im = ax.imshow(data, cmap=\"viridis_r\", norm=norm)\r\n    cb = fig.colorbar(im, ax=ax)\r\n    cb.minorticks_on()\r\n\r\nsfile = \"/tmp/test.png\"\r\nprint(f\"saving to {sfile}\")\r\nfig.savefig(sfile)\r\n```\r\n![test](https://github.com/matplotlib/matplotlib/assets/14075922/0a88b5d1-331c-4d57-9f29-dc95331e972d)\r\n\r\nHere's my attempt to reproduce what's happening internally (within `cb.minorticks_on()`) with a more minimal example\r\n```python\r\nimport matplotlib.ticker as mticker\r\n\r\nlog_t = mticker.LogLocator(base=10, subs=None)\r\nsymlog_t = mticker.SymmetricalLogLocator(linthresh=1, base=10, subs=None)\r\n\r\nprint(log_t.tick_values(vmin=1, vmax=100))\r\nprint(symlog_t.tick_values(vmin=1, vmax=100))\r\n```\r\noutputs\r\n```\r\n[2.e-01 3.e-01 4.e-01 5.e-01 6.e-01 7.e-01 8.e-01 9.e-01 2.e+00 3.e+00\r\n 4.e+00 5.e+00 6.e+00 7.e+00 8.e+00 9.e+00 2.e+01 3.e+01 4.e+01 5.e+01\r\n 6.e+01 7.e+01 8.e+01 9.e+01 2.e+02 3.e+02 4.e+02 5.e+02 6.e+02 7.e+02\r\n 8.e+02 9.e+02 2.e+03 3.e+03 4.e+03 5.e+03 6.e+03 7.e+03 8.e+03 9.e+03]\r\n[  1.  10. 100.]\r\n```\r\n\r\nI think that it is reasonable to expect `SymmetricalLogLocator` and `LogLocator` to produce identica ticks in this case (for values above `linthresh`), and this behaviour could be generalised to any integer base.\r\n\r\n### Proposed solution\r\n\r\nAt the moment I don't have a working patch for this. What I do not understand is how major ticks are sorted out from minor ticks in the case `LogLocator`. With a better understanding of the internals I'm willing to bet that the behaviour I expect should be easy to implement, and I'm more than willing to contribute it, but I may need some guidance.","author":{"url":"https://github.com/neutrinoceros","@type":"Person","name":"neutrinoceros"},"datePublished":"2023-05-28T11:14:38.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/25994/matplotlib/issues/25994"}