APM is in the process of migrating from react-vis to Elastic Charts, and while Elastic Charts is getting a lot of things right, I think two small changes could improve the UX:

• The tooltip should be fixed on the y-axis and only move along along the x-axis. Having it move in both direction is distracting and doesn't add anything
• As a user I expect to see the point on the line highlighted as I move long the x-axis. Unless it hit the exact spot wrt to both the y-axis and x-axis where the line is, it is not highlighted.

Old charts (react-vis)

New charts (Elastic Charts)