feat(chart) :: draw horizontal reference lines

[81reap]

Motivation

#1374

Description

• A row with a yline is drawn as a line across the chart at that value of the y axis, with yline_label and yline_color for its text and its color. Reference lines are rows, so a chart can have as many of them as the query returns. A line follows its axis, so on a horizontal bar chart a yline is drawn down the chart rather than across it. They are not added to the total of a stacked chart, and are not filled in an area chart.
• Document the three parameters, add two examples (an alarm threshold on a line chart, a quota on a horizontal bar chart), and a changelog entry.

Testing

• Two new browser tests: one chart draws a line and a label for every yline row it receives, and a horizontal bar chart draws its yline down the chart rather than across it.

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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 ← this PR
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