Allow plotting multiple means in one graph.

Using the same functionality as the commit where we removed the unused
key suffix, we can admit multiple mean=x.y parameters and plot those
with only minor modifications.

Change-Id: I63d219bb5f04615abeb8282ba7005d2d2fd6227e

Author: georges-arm

lnt/server/ui/views.py

@@ -1102,7 +1102,7 @@ def v4_graph():
                 None))
 
     # Draw mean trend if requested.
-    if mean_parameter:
+    for mean_parameter in mean_parameters:
         machine, field = mean_parameter
         test_name = 'Geometric Mean'