Support custom scale for series in graph.

Currently in a line chart, I have one series that ranges 0 to 100, while another series ranges from 0 to 1 and yet another ranges from -1 to 1. The resulting graph isn't very useful. I could scale and offset the values of the [0,1] range and [-1,1] ranges, but then when the user hovers over a point on that line, they will see the wrong value.

I would either like to be able to specify a range when adding a series and have it scale like the secondary range does (AKA, to have more than just two ranges) or I would like to specify the displayed hover value separately from the rendered value per point (so it might render at 33, but when hovered over show a value of -1).

Start by reading the line-chart handling for the existing secondary range. Compare the two requested directions: custom ranges per series, or separate rendered and hovered values per point. The work is done when the chosen design supports the requested value ranges while showing accurate hover values.

Written by the indexing model from the issue text.