added reference

anushkrishnav · web-flow · commit 0f6adb3f026d · 2020-08-10T01:44:37.000+05:30
diff --git a/examples/lines_bars_and_markers/hat_graph.py b/examples/lines_bars_and_markers/hat_graph.py
@@ -2,7 +2,10 @@
 ===============================
 Hat Graph with labels
 ===============================
-This example shows a how to create a hat graph and how to annotate with labels.
+This example shows a how to create a hat graph
+and how to annotate with labels.
+Refer (https://doi.org/10.1186/s41235-019-0182-3)
+to know more about hat graph 
 """
 import matplotlib
 import numpy as np
