feat(chart) :: draw vertical reference lines

[81reap]

Motivation

#1374

Description

• xline is yline for the x axis
• When time is set, an xline is a date or a timestamp. On a chart with text labels on the x axis, it is one of those labels.
• y_reference_lines generalizes into reference_lines to take in rows of x and y reference lines
• Document the three parameters, add a "Marking events" example (a deploy and an incident on a latency chart), and extend the changelog entry.

Testing

• One new browser test: a time chart draws a line and a label for every xline row it receives.

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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 ← this PR
4. feat(chart) :: turn a reference line into a band #1377 :: feat(chart) :: turn a reference line into a band

[81reap]

rebased. here are screenshots

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