Use a self referencing Many2One as graph x axis

The client group by the value of the field, so it is unique
in contrary of the name.

issue5525
review27441002

view/work_graph.xml

@@ -3,7 +3,7 @@
 this repository contains the full copyright notices and license terms. -->
 <graph type="pie">
     <x>
-        <field name="name"/>
+        <field name="work"/>
     </x>
     <y>
         <field name="duration" timedelta="company_work_time"/>

work.py

@@ -46,6 +46,9 @@ class Work(ModelSQL, ModelView):
         states={
             'readonly': Not(Bool(Eval('active'))),
             })
+    # Self referring field to use for aggregation in graph view
+    work = fields.Function(fields.Many2One('timesheet.work', 'Work'),
+        'get_work')
 
     @classmethod
     def __setup__(cls):
@@ -157,6 +160,9 @@ class Work(ModelSQL, ModelView):
                 durations[work_id] = duration
         return durations
 
+    def get_work(self, name):
+        return self.id
+
     def get_rec_name(self, name):
         if self.origin:
             return self.origin.rec_name