Move more computation to `vega-lite`

[mpickering]

At the moment we do some processing of the data to find the biggest K bands and collapse them together. We also sort the bands in Haskell which makes displaying them in vega-lite easier but it's harder to change the rendering as you have to modify the code generator.

It might be better to just do all the processing in vega-lite, if the performance is not too bad, but we probably don't have massive data sets.