Option to have end points extend from last point with value if they are undefined (`extendEnds`)

[pbeshai]

If the start point is undefined, draw a dotted line flat to the first defined data point.
If the end point is undefined, draw a dotted line from the last defined data point.

Make this a configurable option as extendEnds (boolean).

[pbeshai]

Turns out a boolean is insufficient, since we do not know the xMin and xMax to extend it to. Instead extendEnds takes [xMin, xMax].