Add support for tickRotor plugin

As of Flot 0.8.3, the [tickRotor plugin] (https://github.com/markrcote/flot-tickrotor)  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.

I see in other PRs there's an attempt to standardize how to lookup a plugin, but as they weren't merged I just looked it up via $.plot.plugins