How to highlight a series in a stepped area chart?

[Benjamin M. Hill]

I've got ~30 data series stuffed into my stepped area chart, so some of the colors are similar.  (shades of purple).

When I mouseover, it would be nice to highlight the "selected" series so I can spot it in other columns.

I like the tooltip as-is (showing the currently hovered step cell)

Is there a way to add this with the events?  It feels like it should be an easy thing.  I looked into aggregationTarget, but that seemed to be a "once you selected stuff".

I think what I'm asking is:

How do I do "focusTarget:'series'" for highlighting?