This definitely looks like it'd be useful! However the only option for splitting by day seems to be infinity (which isn't necessarily practical...). Set start/end, and click "Suggest overnight Stops". Tried in both FF and Chrome under Linux. Setting them manually works.
Would it also be possible to divide the altitude graph by day? With points demarking where the overnight stays are for example, and resetting the x-axis to zero for the following day. Or a gap between separate graphs for each day.
Finally an unrelated suggestion (so can make a separate post if it's useful), I often find myself wanting to click/hover on the road types in route summary and have the relevant sections in the route highlighted (for example, I want to see all the parts that are busy roads and investigate to see how scary they look).
And thanks for an excellent website. And apologies if I've missed something obvious and the above is possible already :)