Add support for tickRotor plugin
As of Flot 0.8.3, the tickRotor plugin has to move the tick labels from
opts.ticks to opts.rotatedTicks to avoid a bug. For tooltips, this means
that any content that uses the %y value won't work. Hence, in this
commit we check for use of tickRotor and lookup 'ticks' vs
'rotatedTicks' in the xaxis object as appropriate.