Use the correct access when computing bounded y-values for constrained panning.

Dygraphs was computing the second y-axis using extreme range of the first y-axis. This fixes bug 294, setting panEdgeFraction to non-zero breaks secondary y-axis label computation.

Also add a minor optimization by moving a computation outside the axis loop.
Merge pull request #224 from kberg/i294

Use the correct access when computing bounded y-values for constrained panning.