change graph y-coordinate handling

Assign y-coords in a preparation step, so that drawing step can refer to
y-coords of other graph elements.
fix final tick

Place the final tick too, if it goes right at the end of the axis.

Adjust margins so that the final tick label can fit.
add post events to graph
add vblank events to graph
add not-looping sections to graph

Highlight the timespans, when repaint is not continuously looping.
make sure the last exit_loop gets graphed

If the last exit_loop does not have a enter_loop to follow, the
not_looping period continues to infinity. Make sure it gets graphed.

Invalid timespec as end time is interpreted as infinity.
README: add example output