Support sparse data in y's graph with domain

The value when the domain skip the record should be undefined so the graph has
no point at this ordinate.

issue10072
review337561002

tryton/gui/window/view_form/view/graph_gtk/graph.py

@@ -370,7 +370,6 @@ class Graph(Gtk.DrawingArea):
             self.datas.setdefault(x, {})
             for yfield in self.yfields:
                 key = yfield.get('key', yfield['name'])
-                self.datas[x].setdefault(key, 0.0)
                 if yfield.get('domain'):
                     context = rpc.CONTEXT.copy()
                     context['context'] = context.copy()
@@ -379,6 +378,7 @@ class Graph(Gtk.DrawingArea):
                         context[field] = model[field].get(model)
                     if not PYSONDecoder(context).decode(yfield['domain']):
                         continue
+                self.datas[x].setdefault(key, 0.0)
                 if yfield['name'] == '#':
                     self.datas[x][key] += 1
                 else: