diff --git a/airflow/www/views.py b/airflow/www/views.py
index ce322d7a6287d..8a45a15444b25 100644
--- a/airflow/www/views.py
+++ b/airflow/www/views.py
@@ -2900,7 +2900,6 @@ def duration(self, dag_id, session=None):
 
         y_points = defaultdict(list)
         x_points = defaultdict(list)
-        cumulative_y = defaultdict(list)
 
         task_instances = dag.get_task_instances_before(base_date, num_runs, session=session)
         if task_instances:
@@ -2918,7 +2917,6 @@ def duration(self, dag_id, session=None):
         )
         if dag.partial:
             ti_fails = ti_fails.filter(TaskFail.task_id.in_([t.task_id for t in dag.tasks]))
-
         fails_totals = defaultdict(int)
         for failed_task_instance in ti_fails:
             dict_key = (
@@ -2929,13 +2927,21 @@ def duration(self, dag_id, session=None):
             if failed_task_instance.duration:
                 fails_totals[dict_key] += failed_task_instance.duration
 
-        for task_instance in task_instances:
-            if task_instance.duration:
-                date_time = wwwutils.epoch(task_instance.execution_date)
-                x_points[task_instance.task_id].append(date_time)
-                fails_dict_key = (task_instance.dag_id, task_instance.task_id, task_instance.run_id)
+        # we must group any mapped TIs by dag_id, task_id, run_id
+        mapped_tis = set()
+        tis_grouped = itertools.groupby(task_instances, lambda x: (x.dag_id, x.task_id, x.run_id))
+        for key, tis in tis_grouped:
+            tis = list(tis)
+            duration = sum(x.duration for x in tis if x.duration)
+            if duration:
+                first_ti = tis[0]
+                if first_ti.map_index >= 0:
+                    mapped_tis.add(first_ti.task_id)
+                date_time = wwwutils.epoch(first_ti.execution_date)
+                x_points[first_ti.task_id].append(date_time)
+                fails_dict_key = (first_ti.dag_id, first_ti.task_id, first_ti.run_id)
                 fails_total = fails_totals[fails_dict_key]
-                y_points[task_instance.task_id].append(float(task_instance.duration + fails_total))
+                y_points[first_ti.task_id].append(float(duration + fails_total))
 
         cumulative_y = {k: list(itertools.accumulate(v)) for k, v in y_points.items()}
 
@@ -2951,12 +2957,12 @@ def duration(self, dag_id, session=None):
 
         for task_id in x_points:
             chart.add_serie(
-                name=task_id,
+                name=task_id + '[]' if task_id in mapped_tis else task_id,
                 x=x_points[task_id],
                 y=scale_time_units(y_points[task_id], y_unit),
             )
             cum_chart.add_serie(
-                name=task_id,
+                name=task_id + '[]' if task_id in mapped_tis else task_id,
                 x=x_points[task_id],
                 y=scale_time_units(cumulative_y[task_id], cum_y_unit),
             )