Allow inverting series highlight behavior: change only color/alpha of the hovered series

[juliusv]

When focusing a series by hovering, the focus.alpha setting allows you to de-emphasize all other series by adding transparency to them. I find that effect a bit too visually jarring and would like to achieve the opposite, where only the series that is currently focused gets emphasized somehow, while all others keep their style. Ideally the emphasis wouldn't be alpha-based (since alpha is already 1), but would allow me to change the color or width of the series.

[juliusv]

The one way I got this working with the current uPlot is by remembering the selected series index with a setSeries hook like this:

  let selectedSeriesIdx: number | null = null;

  // ...

      setSeries: [
        (u: uPlot, seriesIdx: number | null, _opts: Series) => {
          if (seriesIdx === selectedSeriesIdx) {
            return;
          }

          if (selectedSeriesIdx !== null) {
            u.series[selectedSeriesIdx].width = 1.5;
          }
          if (seriesIdx !== null) {
            u.series[seriesIdx].width = 2.2;
          }
          selectedSeriesIdx = seriesIdx;
          u.redraw();
        },
      ],

However, the required full redraw makes hovering sluggish when there are many series, so I probably won't be able to use this solution.