chart: reference lines for limits and for events #1374

What are you building with SQLPage ?

Host telemetry dashboards for a fleet of NixOS machines. Metrics are collected to Parquet, queried with DuckDB, and rendered entirely by SQLPage.

What is your problem ? A description of the problem, not the solution you are proposing.

While I am able to graph data, it is often lost without additional context of expected limits, ranges, and events. For example

• a CPU temperature against the part's Tjmax
• a power draw against a supply or circuit budget
• a request latency against an SLO
• a disk usage against a capacity alarm

This also holds along the x-axis. A latency chart shows that something
went wrong at 11:30, but not that 10:00 was a deploy and 11:30 to 13:00 was an
incident. Reading it means keeping a second window open.

While it would also be nice for SQLPage to be able to page/alert me or trigger a webhook, I think it is out of scope for this request. See #1350 and #1351

What are you currently doing ? Since your solution is not implemented in SQLPage currently, what are you doing instead ?

I am doing a mix of different things.

1. Emitting y-lines as an extra series to plot. This kinda works, but breaks with stacked and area charts.
2. Putting threshold context in the title.

Describe the solution you'd like

I would like to be able to pass in any number of x and y reference lines with the option to upgrade reference lines into reference areas by adding more metadata to the request. Changes should be additive so that it is easy to add/remove reference lines from an end user perspective.

Describe alternatives you've considered

1. Emitting y-lines as an extra series to plot. This kinda works, but breaks with stacked and area charts.
2. Putting threshold context in the title.

Additional context

example refrence line :: https://docs.aws.amazon.com/quick/latest/userguide/reference-lines.html