spline: extrapolate beyond last sample

[Dan Jacobson]

I recommend spline(1) have an option to allow extrapolated samples
beyond the end of the data. I.e., we have data for 4:00, 5:00, 6:00,
but would like to know what the value might be at 7:00.
Or the man page should say that this is will never be allowed on purpose.
One can get so fancy with e.g.,
$ echo 0 0 1 0 1 1 0 0 | spline -d 2 -a -s -p -T -14 -n 500 | graph -T X
but one has no way of estimating burger sales at 7:00, or there is a
way but not strikingly documented.