When you highlight a section of the activity, it will “zoom in” on that selection. And it will show the stats for that section regardless what intervals exist.
The data is at the top left of the charts.
I think this satisfies your “so far in the activity” mode, though it takes highlighting the sections you want to analyze. If you’re looking for a singular chart that pre-calculates this for the entire activity, it might be possible with a custom activity chart: Custom activity charts or a custom stream: Custom activity streams with Javascript