Use Arc instead of Vec for cheaper PlotPoints clone

[trueb2]

This PR makes it cheaper to create Line Plots. On each pass, I noticed that the PlotPoints::new interface used to copy 10s-100s of MiB, when we could be doing a cheap Arc<[PlotPoint]> clone instead of an expensive Vec<PlotPoint> clone.

It is related to emilk/egui_plot#18, but as discussion has referenced, there are several aspects degrading rendering performance and this is not a silver bullet.

Construction is slightly different

let line_points =  x.zip(y)
    .map(|(x, y)| [x, y].into())
    .collect::<Vec<_>>()
    .into(); // Use the impl From<Vec<T>> for Arc<[T]>
 let points = vec![line_points];

Usage looks identical.

for series in points {
  ui.line(Line::new(PlotPoints::new(series.clone())));
}

If ExplicitGenerator were Clone, then we would be able to have PlotPoints be Clone.

[emilk]

egui_plot has recently been moved to its own repository, at https://github.com/emilk/egui_plot

This will hopefully speed up its development by having more reviewers and maintainers.

Please re-open this PR at https://github.com/emilk/egui_plot/pulls

See also:

• Looking for maintainer egui_plot#58
• Move egui_plot to its own repo #4828