Setting graph background to transparent.

This allows the white background to show through the Graph,
fixing the background problem in #359

@iXce's change in 94d69f79 was on the right track, but
has to occur earlier in the draw cycle.

printrun/graph.py

@@ -79,8 +79,6 @@ class Graph(BufferedCanvas):
         #b = gc.CreateLinearGradientBrush(0, 0, w, h, col1, col2)
 
         gc.SetPen(wx.Pen(wx.Colour(255, 0, 0, 0), 4))
-        gc.SetBrush(gc.CreateBrush(wx.Brush(wx.Colour(0, 0, 0, 0))))
-        gc.DrawRectangle(0, 0, self.width, self.height)
 
         #gc.SetBrush(wx.Brush(wx.Colour(245, 245, 255, 52)))
 
@@ -238,6 +236,7 @@ class Graph(BufferedCanvas):
         self.Refresh()
 
     def draw(self, dc, w, h):
+        dc.SetBackground(wx.Brush(wx.Colour(0,0,0,0)))
         dc.Clear()
         gc = wx.GraphicsContext.Create(dc)
         self.width = w