Add ridgeline plots

[slowe]

Like the Joy Division/pulsar plot https://www.data-to-viz.com/graph/ridgeline.html

[slowe]

Examples where we have made these include measuring light in our space and our COVID-19 cases by local authority stacked plot.

[slowe]

I've started having a look at this. This is where I'm up to. It is initially derived from the line chart code but with changes to alter the y-axis scaling and positioning. I'm proposing that we add the following ridgeline specific options to the config:

ridgeline:
  height: 2
  curvature: 0.5

where height is scaled by the maximum value to the spacing of ridgelines (1 = no overlap) and curvature is in the range 0 (join with straight lines) to 1 (curvy). For each series we have the additional options of:

• fill to set the fill colour (default #ffffff)
• fillOpacity to set the fill opacity (default 0.8)

[slowe]

A few examples of what can be done: