fix for multi-column legends

made setHideAction() figure out the right text label of the color box in
case of a multi-column legend
optimization for hiding multiple graphs

When hiding multiple graphs at the same time (that is, when using
legend.hidden property), redrawing the canvas for every graph being
hidden is suboptimal. It is legend.hidden.length times faster to redraw
it only once
Merge pull request #9 from campugnatus/my-branch

Couple of fixes