“RE: How Could I Improve this Graph?”

"There are lies, damned lies, and statistics."

Actually you can do all the averaging you want. You can take the mean, or the numerical average, etc. If you're trying to show day-to-day improvement, then you need to display those relationships using only two values <today> / <previousday>.

(today's avg speed X today's miles) / (total avg speed X total avg miles)

It seems to me you would want:

(today's avg speed X today's miles) / (total avg speed X total miles) to keep the same relationship.

Don