grapher: scale from right end of graph

grapher: scale from right end of graph

The right side of the graph represents the most recent time. Since
there is never anything interesting to the right of that, it makes
sense to have the origin of the scaling be there.

* grapher/Graph.hxx (getHorizontalScale): new function
* grapher/GraphStyle.cxx (GraphStyleBar::draw, GraphStyleDot::draw,
  GraphStyleEvent::draw): Use cairo transform functions to set up
  scaling.
  (GraphStyleBar::dataIndexAtPoint, GraphStyleEvent::dataIndexAtPoint):
  Base calculations on scaling from right.