A grouped tip for multi-series plots

[mbostock]

As shown here: https://observablehq.com/@recifs/plot-grouped-tips

[pdebruic]

Would be nice if this could show the interpolated, as drawn, value when data are plotted at unaligned timestamps. https://talk.observablehq.com/t/faceted-multiline-chart-with-tooltips/8720/5

[Fil]

@pdebruic we could open a new discussion about this. By default the interpolation is done by SVG, and Plot only passes path commands to connect the (actual) dots. To find the interpolated values, in the general case, you might need a solver.