diff --git a/plot_transmission_timeline.py b/plot_transmission_timeline.py
index 528b5c1..9ba94b7 100755
--- a/plot_transmission_timeline.py
+++ b/plot_transmission_timeline.py
@@ -192,17 +192,18 @@ if __name__ == "__main__":
 
     print("Calculate and polt CDF...")
     # Get the frequency, PDF and CDF for each value in the series
+    # copy column
+    transmission_df["gp_frequency"] = transmission_df["goodput"]
 
     # Frequency
-    transmission_df["gp_frequency"] = transmission_df["goodput"]
-    transmission_df.groupby("gp_frequency")["gp_frequency"].agg("count").pipe(pd.DataFrame)
+    transmission_df = transmission_df.groupby("gp_frequency")["gp_frequency"].agg("count").pipe(pd.DataFrame)
 
     # PDF
     transmission_df["pdf"] = transmission_df["gp_frequency"] / sum(transmission_df["gp_frequency"])
 
     # CDF
     transmission_df["cdf"] = transmission_df["pdf"].cumsum()
-    transmission_df.reset_index(inplace=True)
+    transmission_df = transmission_df.reset_index()
 
     transmission_df.plot(x="goodput", y=["cdf"], grid=True)