Remove code that clamps horizontal hover to the final point. I don't think

Remove code that clamps horizontal hover to the final point.  I don't think
these do anything; all tests continue to work fine.  This makes it so that
if you have two lines, the first of which continues later than the second,
hovering will work for the first line at points later than the second
ends.

Modify connect-separated test to have nulls at end of final dataset,
to show the bug fix works.