Add plot object in plotHover callback, in order to access plot data
(specially others series)

This is needed by curvedLine plugin which force to have 2 series (one to
display curve, and one to display real point). So label are to be read
on 1 series but event is thrown of the second series. With this patch,
we can access to previous series to get label, in order to display it in
plotHover div