[LNT] Fix plotting issue caused by r188111

[kongyi]

Summary

The bug is introduced in revision 188111, when date is added to graph point metadata. min(enumerate(values)) is used to retrieve both minimum value and its index. However Python prioritize the first element in tuple when sorting, thus the actual value retrieved is the first run value. The fix flips the elements of tuple so that the value is prioritized over run order.

No new regression is introduced because the changed value is local.

lnt/server/ui/views.py

for i,(name,machine,tests,field,agg_fn) in enumerate(graph_parameters):
             metadata = {"label":point_label}
             
             # When we can, map x-axis to revisions, but when that is too hard
             # use the position of the sample instead.
             rev_x = convert_revision(point_label)
             x = rev_x[0] if len(rev_x)==1 else pos
 
             values = [v*normalize_by for v in data]
-            min_index,min_value = min(enumerate(values))
+            min_value,min_index = min((value, index) for (index, value) in enumerate(values))
             metadata["date"] = str(dates[min_index])
             pts.append((x, min_value, metadata))
 
             # Add the individual points, if requested.
             # For each point add a text label for the mouse over.
             if show_all_points:
                 for i,v in enumerate(values):
                     point_metadata = dict(metadata)