Interpolation fails for x=time when data for y has gaps

[angiehjort]

see this line chart

here is the original data in purple and how interpolation result should look like in green

Turns out there is an error in vizabi core:
the field "x" doesn't get interpolated and instead of 1996 1997 1998 1999 2000 it shows 1996 2000 2000 2000 2000

my fix in the commit below is ugly, but it works.
but it's ugly, can it be done more elegant?

the result should look like this then