I'm seeing x0 and x1 NaN values when data contains zeroes e.g.

var data = [
  {id: 'A', val1: 0, val2: 125},
  {id: 'B', val1: 0, val2: 150},
  {id: 'C', val1: 1, val2: 100}
];

If I sum by val2 or use the default tiling strategy, I don't see NaNs.

Example here:

https://bl.ocks.org/peterrcook/21c0582665261a5f9d270b01d1a12d76

Maybe related to #35