fix: continuous colorscale strings with discrete-only chart types

[FBumann]

Summary

• Passing a continuous colorscale string (e.g., colors='turbo') to area(), line(), fast_bar(), box(), or pie() previously failed because resolve_colors set color_continuous_scale, which these Plotly Express functions don't accept.
• Added discrete_only parameter to resolve_colors() that samples from the continuous scale into a color_discrete_sequence instead.
• bar(), scatter(), and imshow() are unchanged (they natively support color_continuous_scale).

Test plan

• All 121 existing tests pass
• Manual smoke test: colors='turbo' works on all 8 chart types

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Added ability to sample continuous color scales into discrete palettes when a specific number of colors is requested.

• Improvements

  • Enhanced color palette handling across line, bar, area, box, and pie charts to respect the actual number of series, producing more consistent and distinguishable discrete color sequences.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 0bf0c574-6e1d-49de-adf1-dd698d87ba78

📥 Commits

Reviewing files that changed from the base of the PR and between fc2fe21 and 23edbc3.

📒 Files selected for processing (2)

• xarray_plotly/common.py
• xarray_plotly/plotting.py

📝 Walkthrough

Walkthrough

Added a helper to sample discrete colors from Plotly colorscales and extended resolve_colors() to accept n_colors (keyword-only). Plotting functions now pass the relevant dimension size as n_colors when resolving colors.

Changes

Cohort / File(s)	Summary
Color sampling & resolver xarray_plotly/common.py	Added _sample_colorscale(name: str, n: int) -> list[str]. Updated `resolve_colors(colors, px_kwargs, *, n_colors: int
Plotting callers xarray_plotly/plotting.py	Updated plotting functions (line, fast_bar, area, box, pie) to compute the color/names dimension size (n_colors) from slots/darray.sizes and pass n_colors=n_colors to resolve_colors() instead of calling it without that parameter.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐇 I nibble at palettes, pick each hue,
Sampling rainbows, one, two, and few,
Plots bloom with counted colors bright,
Hop-scotch through scales, a tidy sight,
A rabbit’s cheer for every view 🥕✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately and specifically describes the main fix: handling continuous colorscale strings with chart types that only support discrete colors.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/continuous-colorscale-discrete-charts

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Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@xarray_plotly/common.py`:
- Around line 275-278: Update the docstring for the discrete_only parameter to
remove "bar" from the list of chart types that sample continuous colors into a
discrete sequence; specifically, edit the text near the discrete_only
description in xarray_plotly/common.py so it only lists line(), area(), box(),
and pie() (since bar() calls resolve_colors(colors, px_kwargs) without
discrete_only=True). Leave the rest of the description unchanged.
- Around line 254-258: The _sample_colorscale function is sampling a fixed n
(default 10) which causes color clustering for small categorical dims; change
resolve_colors so sampling uses the actual category count: either call
resolve_colors/_sample_colorscale after assign_slots so you can read the
cardinality from darray.sizes[color_dim], or refactor resolve_colors to accept
the resolved darray and the color dimension name and compute n =
int(darray.sizes[color_dim]) before calling _sample_colorscale; update all call
sites (including the calls that currently pass only colors when
discrete_only=True) to provide the proper n or the darray+color dim so samples
span the full colorscale.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: f275bb3a-1782-4698-9ac8-ed1ee7b15780

📥 Commits

Reviewing files that changed from the base of the PR and between 63f37a7 and 6a971a3.

📒 Files selected for processing (2)

• xarray_plotly/common.py
• xarray_plotly/plotting.py