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, 9 deletions

diff --git a/grapher/Graph.cxx b/grapher/Graph.cxx
index afb52c74..4d697ae6 100644
--- a/grapher/Graph.cxx
+++ b/grapher/Graph.cxx
@@ -47,8 +47,7 @@ void Graph::draw(Cairo::RefPtr<Cairo::Context> cr)
       _left = _right - 5000;
     }
   cr->save();
-  double horizScale
-    = _zoomFactor * _graphWidth / static_cast<double>(_right - _left);
+  double horizScale = getHorizontalScale();
   cr->translate(_xOffset, _yOffset);
   cr->set_line_width(_lineWidth);
 
@@ -103,10 +102,11 @@ void Graph::draw(Cairo::RefPtr<Cairo::Context> cr)
   cr->select_font_face("Sans", Cairo::FONT_SLANT_NORMAL,
                        Cairo::FONT_WEIGHT_NORMAL);
   cr->set_font_size(10.0);
-  cr->move_to(_xOffset, _yOffset);
-  cr->line_to(_xOffset, _height);
-  cr->move_to(_xOffset, _graphHeight);
-  cr->line_to(_graphWidth, _graphHeight);
+  cr->translate(_xOffset, 0.0);
+  cr->move_to(0.0, _yOffset);
+  cr->line_to(0.0, _height);
+  cr->move_to(0.0, _graphHeight);
+  cr->line_to(_graphWidth - _xOffset, _graphHeight);
   cr->stroke();
   std::valarray<double> dash(1);
   dash[0] = _graphHeight / 10;
@@ -114,7 +114,7 @@ void Graph::draw(Cairo::RefPtr<Cairo::Context> cr)
   double prevTextAdvance = 0;
   for (int64_t tickVal = startTime; tickVal <= _right; tickVal += majorUnit)
     {
-      double x = (tickVal - _left) * horizScale + _xOffset;
+      double x = (tickVal - _right) * horizScale + _graphWidth;
       cr->move_to(x, _yOffset);
       cr->line_to(x, _graphHeight);
       std::ostringstream stream;
@@ -170,8 +170,7 @@ bool Graph::containsPoint(double x, double y)
 
 int64_t Graph::getTimeAtPoint(double x)
 {
-  return (_left
-          + (_right - _left) * ((x - _xOffset)/(_zoomFactor * _graphWidth)));
+  return (x - _graphWidth) / getHorizontalScale() + _right;
 }
 
 void Graph::window2GraphCoords(double x, double y,