analytics: Add backend for messages read over time graph.

This commit includes changes in the api /json/analytics/chart_data to
send data for the newly added graph, as well as tests.

analytics/tests/test_views.py

@@ -181,6 +181,23 @@ class TestGetChartData(ZulipTestCase):
             'result': 'success',
         })
 
+    def test_messages_read_over_time(self) -> None:
+        stat = COUNT_STATS['messages_read::hour']
+        self.insert_data(stat, [None], [])
+        result = self.client_get('/json/analytics/chart_data',
+                                 {'chart_name': 'messages_read_over_time'})
+        self.assert_json_success(result)
+        data = result.json()
+        self.assertEqual(data, {
+            'msg': '',
+            'end_times': [datetime_to_timestamp(dt) for dt in self.end_times_hour],
+            'frequency': CountStat.HOUR,
+            'everyone': {'read': self.data(100)},
+            'user': {'read': self.data(0)},
+            'display_order': None,
+            'result': 'success',
+        })
+
     def test_include_empty_subgroups(self) -> None:
         FillState.objects.create(
             property='realm_active_humans::day', end_time=self.end_times_day[0],

analytics/views.py

@@ -236,6 +236,12 @@ def get_chart_data(request: HttpRequest, user_profile: UserProfile, chart_name:
                              {str(id): name for id, name in Client.objects.values_list('id', 'name')}}
         labels_sort_function = sort_client_labels
         include_empty_subgroups = False
+    elif chart_name == 'messages_read_over_time':
+        stats = [COUNT_STATS['messages_read::hour']]
+        tables = [aggregate_table, UserCount]
+        subgroup_to_label = {stats[0]: {None: 'read'}}
+        labels_sort_function = None
+        include_empty_subgroups = True
     else:
         raise JsonableError(_("Unknown chart name: %s") % (chart_name,))