Prevent slider from cutting off series.

This fixes what I think is a bug:

I have 4 series,

[
  [{0, 1}, {1, 1}],
  [{1, 1}, {2, 1}],
  [{2, 1}, {3, 1}],
  [{3, 1}, {4, 1}]
]

I use a slider to select the x-range (2.5, 4)

This means stackedData ends up being

[ [], [], [], [[3,4], [1, 1]] ]

At this point, reading stackedData[0][0] produces undefined, even though
there is data that belongs to the range.