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.
Fix wrong values in legend when using stackedGraph with missing values
Consider all the series when searching previous stacked one and use Dygraph.isValidPoint to validate the point (not just !yval)
See http://jsfiddle.net/6W7EQ/2/
Merge pull request #224 from kberg/i294

Use the correct access when computing bounded y-values for constrained panning.
Merge pull request #225 from vlsi/master

Fix wrong values in legend when using stackedGraph with missing values
Merge branch 'master' of https://github.com/danvk/dygraphs