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Enable logscale Colorbar#7894

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bmaranville wants to merge 8 commits into
plotly:masterfrom
bmaranville:colorbar-logscale
Open

Enable logscale Colorbar#7894
bmaranville wants to merge 8 commits into
plotly:masterfrom
bmaranville:colorbar-logscale

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@bmaranville

@bmaranville bmaranville commented Jul 8, 2026 •
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Implement logscale colorbars natively

Motivation

This feature has been requested a number of times (see #7376 and https://community.plotly.com/t/logarithmic-coloraxis-in-heatmaps-not-working-as-documented/90650/5?u=matinh, for example) and our organization uses logarithmic scaling almost exclusively as the default scale for colormaps for heatmap plots, but it is currently awkward to have to manually set tick labels as suggested in the docs here:
https://plotly.com/python/colorscales/#customizing-tick-text-on-logarithmic-color-bars

Implementation

This PR

  • modifies attribute colorbar.type to be an enum with values ['linear', 'log'], defaulting to 'linear'
  • adds scaling to the range of the colorbar when 'log' is the type
  • adds tests for
    • validation of new attribute
    • proper tick labels in 'log' mode
    • proper color mappings, including for out-of-range (e.g. negative)

Note
this PR was prepared with the assistance of Gemini AI (but audited completely by the human me). The AI was particularly helpful in writing tests.

With this PR, the following trace:

{
  "type": "heatmap",
  "z": [
    [
      1,
      3.1622776601683795,
      10,
      31.622776601683793,
      100,
      316.22776601683796,
      1000
    ],
    [
      3.1622776601683795,
      10,
      31.622776601683793,
      100,
      316.22776601683796,
      1000,
      3162.2776601683795
    ],
    [
      10,
      31.622776601683793,
      100,
      316.22776601683796,
      1000,
      3162.2776601683795,
      10000
    ],
    [
      31.622776601683793,
      100,
      316.22776601683796,
      1000,
      3162.2776601683795,
      10000,
      31622.776601683792
    ],
    [
      100,
      316.22776601683796,
      1000,
      3162.2776601683795,
      10000,
      31622.776601683792,
      100000
    ],
    [
      316.22776601683796,
      1000,
      3162.2776601683795,
      10000,
      31622.776601683792,
      100000,
      316227.7660168379
    ],
    [
      1000,
      3162.2776601683795,
      10000,
      31622.776601683792,
      100000,
      316227.7660168379,
      1000000
    ]
  ],
  "colorscale": "Viridis",
  "colorbar": {
    "type": "log",
    "title": "Log Scale",
    "tickmode": "auto"
  }
}

renders like this:
image

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@bmaranville