We need to take into account the real start of the capture to
land the ticks in the right position. Add "epoch" concept
to SpVisualizerTicks for just this purpose.
We need some sort of scale for content, so we will do this with
an overlay for now. However, we will also want something to be
able to do selections in the future.
This still needs some iteration for correctness, but this sort
of gets the ball rolling.
