[Lens][ColorMapping] Auto Assignment breaks for gradient with 1 or more assignments

[nickofthyme]

Kibana version:

latest

Describe the bug:

When assigning a gradient palette to terms in a Lens chart using the new color mapping UI, assigning an auto assignment incorrectly caps the color loop at the number of auto assignments instead of the total assignments.

Zight.Recording.2025-01-31.at.01.14.12.PM.mp4

Steps to reproduce:

1. Create a bar chart with a y metric and a breakdown by.
2. Open the palette settings and select the new color mapping mode.
3. Click the gradient option. Do not create an assignment yet.
4. Notice the colors are correctly assigned.
5. Add one assignment and leave it blank (i.e. auto assignment)
6. Notice all series are colored the same, the first color.
7. Now add another auto assignment.
8. Notice the series are looping every other color.

Expected behavior:

Auto assignments should be colored correctly according to the defined gradient.

Any additional context:

This code is to blame...

kibana/src/platform/packages/shared/kbn-coloring/src/shared_components/color_mapping/color/color_handling.ts

Lines 110 to 112 in 1b61f38

	const totalColorsIfGradient = assignments.length || notAssignedCategories.length;
	const indexIfGradient =
	(nonAssignedCategoryIndex - autoByOrderAssignments.length) % totalColorsIfGradient;

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)

[nickofthyme]

After closer inspection I believe this is expected behavior.

Zight.Recording.2025-01-31.at.01.54.01.PM.mp4

There is however a major difference in how this Color for unassigned terms behavior works between the categorical mode and gradient mode. The main difference being that the number of assignment on the gradient mode determines the extent of the color scale. So in gradient mode, if you have 2 auto assignments with 4 categories/series in the chart, the scale will start looping after the 2nd color assignment. This is not the case with categorical palettes as their colors are predetermined.

I wonder if we can improve this behavior because it really feels like a bug.

What if we had a separate input to dictate the total steps of the steps in gradient. This could have an auto mode that updates to the number of assignments as it is now, or the user can change this to the desired count.

cc: @markov00 @MichaelMarcialis