[AIRFLOW-225] Better units for task duration graph

Right now the job duration window defaults to hours, which for short lived tasks
results in numbers out to five decimals. This patch adjusts the scale of the Y-axis
in accordance with the maximum value of the durations to be shown.

airflow/www/views.py

@@ -1477,18 +1477,35 @@ def duration(self):
                 include_downstream=False)
 
         all_data = []
+        max_duration = 0
         for task in dag.tasks:
             data = []
             for ti in task.get_task_instances(session, start_date=min_date,
                                               end_date=base_date):
                 if ti.duration:
                     data.append([
                         ti.execution_date.isoformat(),
-                        float(ti.duration) / (60*60)
+                        ti.duration
                     ])
+                    if max_duration < ti.duration:
+                        max_duration = ti.duration
             if data:
                 all_data.append({'data': data, 'name': task.task_id})
 
+        def divide_durations(all_data, denom):
+            for data in all_data:
+                for d in data['data']:
+                    d[1] /= denom
+
+        if 60*60 < max_duration:
+            unit = 'hours'
+            divide_durations(all_data, float(60*60))
+        elif 60 < max_duration:
+            unit = 'minutes'
+            divide_durations(all_data, 60.0)
+        else:
+            unit = 'seconds'
+
         tis = dag.get_task_instances(
                 session, start_date=min_date, end_date=base_date)
         dates = sorted(list({ti.execution_date for ti in tis}))
@@ -1503,7 +1520,7 @@ def duration(self):
             'airflow/chart.html',
             dag=dag,
             data=json.dumps(all_data),
-            chart_options={'yAxis': {'title': {'text': 'hours'}}},
+            chart_options={'yAxis': {'title': {'text': unit}}},
             height="700px",
             demo_mode=conf.getboolean('webserver', 'demo_mode'),
             root=root,