Chart tooltip no longer shows the values for all repos at a point

[lpil]

Previously a tooltip would show the values for all repos at a point in time that the mouse hovered over. The new version only shows a single language, making comparison impossible.

[timqian]

It is actually a bug for the old chart, the x axis for different repos are usually different

ps. I deployed another version of star history using the previous chart lib
https://old-star-history.t9t.io/
if you prefer the old chart style, you can try this one.