A look at data entry in CRUCS. The top section is for putting in additional control points. There's another section that lets you specify an end point. If you don't do that, the point with the largest time is used as the end point, but you can also extrapolate from the curve or set an earlier end time. The control points section lets you remove points (which sticks the data back into the insert section for ease of undo/editing that point).
The time fields are kind of neat because rather than try to be rigid with the format, if there's no : the number gets interpreted as seconds. If there is a :, everything to the left gets treated as minutes and everything to the right gets treated as seconds, which means if you want to add 30 seconds to a time like 8:47, you could put in 8.5:47 (8 and a half minutes and 47 seconds) and it'll normalize to the right thing, or similarly 8:77 and again it'll do the right thing.
I'm going to add a side bar element to the left of that section listing different data series so it's convenient to switch among multiple curves, but otherwise I think that section is basically finished.