Changes goodput calculation.

plot_transmission_timeline.py

@@ -147,7 +147,7 @@ if __name__ == "__main__":
 
     print(transmission_df)
 
-    range_start_time = transmission_df.index.min()
+    range_start_time = str(transmission_df.index.min())
     range_sum_interval = "{}s".format(args.interval)
 
     # create timedelta range with maximum timedelta