Make setSelection() work with missing points.

If the points array is missing points, for instance because the data
handler omitted them, setSelection() will now still properly find
the points that are being selected by looking at the points' idx
property instead of just indexing into the points array.
this.getLeftBoundary_() just returns the number of points that were omitted from the points array due to x-axis display constraints.  It's only necessary if you're computing the index rather than finding the point, so it's not needed in the alternative method.

I switched it to check if the common case is actually correct, and use the fast path in that case.