Add support for a new TIMELINE style for drawing a data-set.

For the TIMELINE style, each (X,Y) pair represents (start,stop) time
in the X dimension for a timeline bar to be drawn. The recently added
"_named" variant of add_data is also useful for supplying a name to be
drawn on the bar.

With multiple timelines within a chart, each one is positioned at a
different integer position along the Y axis, (and the Y axis range
computation is changed to preserve this).

There's only minimal cleverness to avoid collision of timeline label,
(the vertical position is cycled unconditionally).

More clever approaches would not change the vertical position if there
is no possibility of conflict, and would also reset to the top
position as soon as possible.

Other potential improvements to the TIMELINE style include:

  * Ensuring that a timeline label is visible whenever any portion of
    a timeline bar is visible.

  * Eliminating the rendering of numbers and ticks on the Y axis.