Support dashed lines in plots

[jleibs]

Would be nice to support dashed lines in plots.

One aspect that comes up when discussing dashed (and possibly dotted) lines is how to parameterize the dash and/or dot spacing.

The SVG approach to this seems nicely general, but will require some additional work on egui_plots.

• https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/stroke-dasharray

[emilk]

This is the current level of support in egui_plot:

https://docs.rs/egui_plot/latest/egui_plot/enum.LineStyle.html