René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: fig.colorbar() requires plt.cm.ScalarMappable() object instead of matplotlib.colors.ListedColormap() as it lacks .get_cmap()","articleBody":"### Bug summary\n\nHi,\n\nI wonder why always people specify a list o colors but then apply `np.linspace(0, 1, N)` over it. This then shifts the colors a bit and some colors are then unused, for example a color between `-4` and `-5`.\n\nI want to avoid the https://gist.github.com/jakevdp/91077b0cae40f8f8244a approach because I already have a series of colors as hex values.\n\nAlthough my testcase is a bit overcomplicated it shows that ticks and color indices are not aligned to the respective square/rectangle filled with the color in the colrbar.\n\nI think this stems from the `_figure.colorbar(plt.cm.ScalarMappable(norm=matplotlib.colors.Normalize(- 11, 11, 11 * 2 + 1), cmap=_cmap))` line.\n\nHere is output from the testcase code.\n\nThank you.\n\n### Code for reproduction\n\n```Python\nimport matplotlib\nimport matplotlib.pyplot as plt\nimport matplotlib.ticker as ticker\n\nimport numpy as np\nimport random\n\n_colors = [\"#930000\", \"#930000\", \"#930000\", \"#930000\", \"#930000\", \"#930000\", \"#960000\", \"#580041\", \"#8110ff\", \"#c500ff\", \"#ff00fd\", \"#eea1d0\", \"#CC79A7\", \"#cc0000\", \"#ff0000\", \"#ff4f00\", \"#ff7c7c\", \"#ff9999\", \"#c58a24\", \"#9c644b\", \"#ffff00\", \"#ffcc00\", \"#cccc00\", \"#7DCCFF\", \"#0042ff\", \"#0000ff\", \"#D6D6D6\", \"#B7B7B7\", \"#8B8B8B\", \"#97CE2F\", \"#219f11\", \"#00ff04\", \"#bbff00\", \"#930000\", \"#930000\", \"#930000\", \"#930000\", \"#930000\", \"#930000\"]\n_cmap = matplotlib.colors.ListedColormap(_colors, \"my_listing\", len(_colors))\n\n_aminoacids = ['C', 'R', 'K', 'E', 'Q', 'D', 'N', 'T', 'S', 'H', 'M', 'P', 'W', 'Y', 'F', 'V', 'L', 'I', 'A', 'G']\n_figure, (_ax1, _ax3, _ax4) = plt.subplots(1, 3, figsize=(16, 9), width_ratios=[55, 1, 6])\n\n_ax1.set_xlim(317, 567)\n_ax1.xaxis.set_major_locator(ticker.MultipleLocator(10))\n_ax1.xaxis.set_minor_locator(ticker.MultipleLocator(5))\n\n_ax1.xaxis.set_tick_params(labelsize=14)\n_ax1.tick_params(axis='x', which='both', labelsize=14)\n_y_ticks = np.arange(len(_aminoacids))\n_ax1.set_yticks(_y_ticks)\n_ax1.set_yticklabels(_aminoacids, fontsize=8)\n_ax1.tick_params(axis='y', which='major', labelsize=8) # minor labels do not appear somehow\n\n_ax1.grid(True, linestyle='--', alpha=0.3, color='gray')\n\n_ax2 = _ax1.twinx()\n_ax2.set_xlim(317, 567) # start X-axis from 1, not zero\n_ax2.set_ylim(0, 1)\n\n_ax2.set_ylabel(f'2nd Y-axis label', fontsize=12)\n_ax1.figure.canvas.draw()\n_ax2.figure.canvas.draw()\n\n_ax3.xaxis.set_major_locator(ticker.MultipleLocator(2))\n_ticks = [int(x) for x in range(- 11, 11)]\n_colorbar = _figure.colorbar(plt.cm.ScalarMappable(norm=matplotlib.colors.Normalize(- 11, 11, 11 * 2 + 1), cmap=_cmap), ticks=_ticks, cax=_ax3, label=\"Colorbar values are not aligned to center of the respective color\", location='right', pad=-0.1, alpha=0.5)\n\nfor label in _ax1.get_xticklabels():\n    label.set_rotation(90)\n    label.set_ha(\"center\")\n\n# does not work\n# _colorbar = _figure.colorbar(mappable=_cmap, ax=_ax1, label=\"values\", location='left', pad=0.15)\n\n_data = []\nfor _x in range(317, 567):\n    _y = _aminoacids[random.randint(0, len(_aminoacids) - 1)]\n    _size = np.random.rand()\n    _color = _colors[random.randint(0, len(_colors) - 1)]\n    print(\"%s %s %s %s\" % (_x, _y, _size, _color))\n    _data.append((_x, _y, float(np.abs(_size) * 5000 * np.random.rand()), 'circle_x', _color, 0.5))\n\n_mpl_scatterplot = _ax1.scatter([x[0] for x in _data], [x[1] for x in _data], s=[x[2] for x in _data], color=[x[4] for x in _data], alpha=0.5, cmap=_cmap)\n\n# draw legends in scale with _ax1\nhandles, labels = [], []\n_junk = 'NNN'\n\nfor _freq in [0.001, 0.01, 0.1, 0.3]:\n    handle = _ax2.scatter(_size, - 400 + _freq, s=float(_freq * 5000), color='gray', alpha=0.5, label=f'Frequency {_freq:.1%}')\n    label = str(_freq)\n    handles.append(handle)\n    labels.append(label)\n\n_ax4.set_axis_off()\n_ax2.legend(loc='upper center', bbox_to_anchor=(1.30, 1.00), labelspacing=3, frameon=False, handletextpad=1.5)\n\nplt.show()\nplt.clf()\n```\n\n### Actual outcome\n\n![Image](https://github.com/user-attachments/assets/5e4647e8-7d5c-4396-8733-cb570bafe7c2)\n\n### Expected outcome\n\nThe ticks and labels should be aligned vertically in the middle of each rectangle filled by some color.\n\n### Additional information\n\n_No response_\n\n### Operating system\n\nGentoo\n\n### Matplotlib Version\n\n3.10.8\n\n### Matplotlib Backend\n\nqtagg\n\n### Python version\n\n3.13.11\n\n### Jupyter version\n\n_No response_\n\n### Installation\n\nNone","author":{"url":"https://github.com/mmokrejs","@type":"Person","name":"mmokrejs"},"datePublished":"2026-02-19T17:28:49.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":30},"url":"https://github.com/31178/matplotlib/issues/31178"}