Feature request: plot multiple values of 1 data type in a chart

[luc-tielen]

Hi,

Is it possible to add possibility to plot multiple values from a single data type.
For example, I'm working on a toy example that shows history of git diffs over time.
Data type is as follows:

type alias Delta =
  { author : Author
  , commit : Commit
  , summary : Summary
  , time : Time
  , additions : Int
  , deletions : Int
  }

But viewCustom does not allow this to be plotted directly, it expects x and y.
So I have to first make 2 lists containing (Time, Float) for the additions and deletions.

...
chartConfig : GraphModel -> LineChart.Config (Time.Time, Int) GraphMsg
chartConfig model =
  { x = Axis.time 1270 "Time" Tuple.first
  , y = Axis.default 450 "LOC" (Tuple.second >> toFloat)
...

This is all fine for plotting, but when I try to show hover data.. most information is now lost.
I have a workaround, but it involves me keeping track of a Dict Time Delta and it's noticably slower for large repositories.

Ideally, it should be possible to have chartConfig : GraphModel -> LineChart.Config Delta GraphMsg;
giving me full access to the data.
Any chance this could be integrated into the library? Or does anyone know a better way of dealing with this kind of data?