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.

1 files changed, 8 insertions, 0 deletions

diff --git a/grapher/Graph.hxx b/grapher/Graph.hxx
index d9f615b3..0853d988 100644
--- a/grapher/Graph.hxx
+++ b/grapher/Graph.hxx
@@ -51,6 +51,14 @@ public:
   int64_t getTimeAtPoint(double x);
   void window2GraphCoords(double x, double y, double& xgraph, double& ygraph);
   static void setCurrentTime(int64_t time) { _currentTime = time; }
+
+  /*
+   * universal horizontal factor
+   */
+  double getHorizontalScale()
+  {
+    return _zoomFactor * _graphWidth / static_cast<double>(_right - _left);
+  }
 protected:
   GraphDataList _datasets;
   int64_t _left;