Labels are provided via a 'label' option on the axis, and can be styled
with the flot-axis-label class.  The labelFont option works similarly to
tickFont, as an override for the default font or the flot-axis-label
class.  The labelPadding option adds extra space between the axis and
its label.

Since most plots with axis labels currently use @markrcote's
flot-axislabels plugin, we also support the axisLabel and
axisLabelPadding options, and the axisLabels / axis[name]Label CSS
classes, to make it as easy as possible to transition from that plugin.
These are deprecated, and will be removed in 1.0.

The implementation uses the internal text API introduced in 0.8.