[FIX] board: height of graph

When a graph is added to the dashboard, it doesn't have a minimum
height set, so only the label and the axis are displayed, not the
content.

opw-766454

addons/board/static/src/less/dashboard.less

@@ -114,6 +114,11 @@
             .o_kanban_view .o_kanban_record .oe_kanban_list_many2many .o_form_field_many2manytags {
                 border: none;
             }
+
+            // Override height of graph. min-height doesn't do the trick
+            .o_graph {
+                height: 500px;
+            }
         }
     }
 }