feat(chart) :: turn a reference line into a band

[81reap]

Motivation

#1374

Description

• Adding yline_end or xline_end to a reference row turns its line into a band that reaches to that value. Label and color work the same as on a line.
• Update the two documentation examples — the throttling threshold becomes a band up to ymax, and the incident on the latency chart covers its whole duration — and extend the changelog entry.

Testing

• The two reference-line browser tests now each check both shapes: one line and one band on the y axis, and one line and one band on the x axis, with both labels visible.

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1. fix(chart) :: line series up on a category axis for every chart type #1371 :: fix(chart) :: line series up on a category axis for every chart type
2. feat(chart) :: draw horizontal reference lines #1375 :: feat(chart) :: draw horizontal reference lines
3. feat(chart) :: draw vertical reference lines #1376 :: feat(chart) :: draw vertical reference lines
4. feat(chart) :: turn a reference line into a band #1377 :: feat(chart) :: turn a reference line into a band ← this PR

[81reap]

@lovasoa seems like we accidentally pulled in a duplicated change log when mergeing mainline into #1401 . I generally find that always rebasing instead of mergeing avoids these kinds of issues. always happy to rebase my PRs and push back up as well :)