Line height is a core font property - it can be specified as part of the
unified 'font' definition - and we therefore need to give users access
to it when they're manually defining the font.
The purpose of the stylesheet hack was to provide a default without
having to use inline styles on containers. We can do this much more
neatly by instead just giving the inline styles to a parent container,
leaving users free to customize the children.
The axis color now controls the color of the axis line, instead of its
ticks and labels, while the tickColor controls the tick color. This
makes a little more sense and provides the minor feature of being able
to change the axis line color separately from that of its ticks. Pull
request #917 ought to be easier to merge now, too.