Add a dpi parameter to render_to_png

pygal/ghost.py

@@ -103,10 +103,11 @@ class Ghost(object):
         with io.open(filename, 'w', encoding='utf-8') as f:
             f.write(self.render(is_unicode=True))
 
-    def render_to_png(self, filename=None):
+    def render_to_png(self, filename=None, dpi=72):
         """Render the graph, convert it to png and write it to filename"""
         import cairosvg
-        return cairosvg.svg2png(bytestring=self.render(), write_to=filename)
+        return cairosvg.svg2png(
+            bytestring=self.render(), write_to=filename, dpi=dpi)
 
     def _repr_png_(self):
         """Display png in IPython notebook"""