Fix a bug in the graphs/index controller: the format is given by the page extension.

controllers/graphs.py

@@ -87,5 +87,7 @@ def index():
     do_title(ax, db, selector)
 
     # delegate the rendering to the view
-    fmt = "svg" if selector.format == "html" else selector.format
+    extension = request.extension
+    fmt = "svg" if extension == "html" else extension
+
     return dict(data=savefig(ax.get_figure(), fmt))