benchmark: add lines to scatter plots

Adds lines between the points of the same category in scatter.R plots.

PR-URL: https://github.com/nodejs/node/pull/22074


Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>