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.

---

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

[lovasoa]

I think we can remove support for yline colors in this pr, then separately add support for both individual point colors and yline colors. What do you think ?

[lovasoa]

Don't you think it would be better to use common property names ? color and label that describe the color and the label of the element being drawn, be it a point, a vertical line, a horizontal line, or something else ?

[81reap]

@lovasoa I can try to split out the colour part into a separate PR, but I would push back against that and your comment a bit. Currently as the PR stands, users are able to add any number of X or Y reference lines. We would need colour to be able to tell them apart.

https://github.com/sqlpage/SQLPage/pull/1375/changes#diff-5caa0e48a864e277cf43a80229ed27f3f626e3be5af47fe03f57ff2e27a58105R807-R820

Your comment did bring to mind accessiblity, mainly colour blindness. I can do a follow up PR to add different kinds of dashed reference lines so that end users don't only have to rely on colour to be able to tell the difference.