diff --git a/graph.py b/graph.py
index 9b0ccd7..e77ad80 100755
--- a/graph.py
+++ b/graph.py
@@ -43,6 +43,7 @@ plt.plot(datenums, data_frame.tempCoolSide2C, label='Cool 2', color='navy', mark
 for i, j in zip(datenums, data_frame.tempCoolSide2C):
     ax.annotate(str(round(j, 2)), xy=(i, j), rotation=45)
 
+plt.axhline(y=-223, color='darkblue', linestyle=':', label='Operating temperature')
 
 plt.legend(bbox_to_anchor=(1, 1), loc="upper left")
 plt.show()