Resolve "feat: Support multiple y values per x value on discrete scatterplot"

This MR ensures that we can render multiple y axis values for a given x axis value in the discrete scatter chart.

Primarily, it changes the way we compute tooltip position which used to be based on the x axis pointer (which works well in case there's only one y axis for a given x coordinate)

Screencasts

Before	After
discrete_scatterplot_before	discrete_scatterplot_after

Closes #392 (closed)