Sort stacked GLQL chart tooltip rows by value

What does this MR do and why?

Stacked column and stacked area charts in GLQL now list tooltip rows by value, largest first, within each stack.

Before, rows came in raw series order. That is the bottom segment of the stack first, so the tooltip read in no useful order.

Bar charts and unstacked charts (line, unstacked column) keep series order.

Example from the DAP impact dashboard, "Credits over time", Sep 11:

Before	After
software_development 115.82	code_review 210.71
code_review 210.71	software_development 115.82
convert_to_gitlab_ci 108.37	Other (6) 114.91
code_review/v1 36.05	convert_to_gitlab_ci 108.37
agentic_chat/v1 27.53	code_review/v1 36.05
Other (6) 114.91	agentic_chat/v1 27.53

Why rows may not match segment positions

ECharts stacks every column in the same series order. So in some columns, the tooltip rows will not line up with the segment positions. We accept this on purpose. The color swatch links each row to its segment.

References

There is no related issue.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open a dashboard with a stacked GLQL column chart, for example DAP impact "Credits over time".
2. Hover a column.
3. Check that the tooltip rows are listed largest first.
4. Open a bar chart or an unstacked chart (line, unstacked column) and check that its tooltip rows keep series order.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.

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