I agree with BrowserUK, use a line( use the smooth option), it should be fast to update. Additionally, break your day into smaller periods, say hours, and seamlessly display them side-by-side on the canvas. That way, each update would only need to rebuild that hour's line segments. An hour of seconds would be 3600 segments. If that is too slow, break it into tenth's of an hour.

See ztk-roller-coaster-simulation for a demo of how to make the segments.