pandas boxplot contains content of plot saved before

I'm plotting some columns of a datafame into a boxplot. Sofar, no problem. As seen below I wrote some stuff and it works. BUT: the second plot contains the plot of the first plot, too. So as you can see I tried it with "= None" or "del value", but it does not work. Putting the plot function outside also don't solves the problem.

Whats wrong with my code?

Edit: here an executable example

 import pandas as pd 
 
 d1 = {'ff_opt_time': [10, 20, 11, 5, 15 , 13, 19, 25  ], 'ff_count_opt': [30, 40, 45, 29, 35,38,32,41]}
 df1 = pd.DataFrame(data=d1)
 d2 = {'ff_opt_time': [1, 2, 1, 5, 1 , 1, 4, 5  ], 'ff_count_opt': [3, 4, 4, 9, 5,3, 2,4]}
 df2 = pd.DataFrame(data=d2)
 
 def evaluate2(df1, df2):
    
     def plot(df, output ):
         boxplot = df.boxplot(rot=45,fontsize=5)
         fig = boxplot.get_figure()
         fig.savefig(output + ".pdf")
     
     df_ot = pd.DataFrame(columns=['opt_time1' , 'opt_time2'])
     df_ot['opt_time1'] = df1['ff_opt_time']
     df_ot['opt_time2'] = df2['ff_opt_time']
     plot(df_ot, "bp_opt_time")
 
     df_op = pd.DataFrame(columns=['count_opt1' , 'count_opt2'])
     df_op['count_opt1'] = df1['ff_count_opt']
     df_op['count_opt2'] = df2['ff_count_opt']
     plot(df_op, "bp_count_opt_perm")    
 
 evaluate2(df1, df2)

Edit: here anohter executeable example. I even used other variable names.

import pandas as pd 

d1 = {'ff_opt_time': [10, 20, 11, 5, 15 , 13, 19, 25  ], 'ff_count_opt': [30, 40, 45, 29, 35,38,32,41]}
df1 = pd.DataFrame(data=d1)
d2 = {'ff_opt_time': [1, 2, 1, 5, 1 , 1, 4, 5  ], 'ff_count_opt': [3, 4, 4, 9, 5,3, 2,4]}
df2 = pd.DataFrame(data=d2)

def evaluate2(df1, df2):

    df_ot = pd.DataFrame(columns=['opt_time1' , 'opt_time2'])
    df_ot['opt_time1'] = df1['ff_opt_time']
    df_ot['opt_time2'] = df2['ff_opt_time']
    boxplot1 = df_ot.boxplot(rot=45,fontsize=5)
    fig1 = boxplot1.get_figure()
    fig1.savefig( "bp_opt_time.pdf")

    df_op = pd.DataFrame(columns=['count_opt1' , 'count_opt2'])
    df_op['count_opt1'] = df1['ff_count_opt']
    df_op['count_opt2'] = df2['ff_count_opt']
    boxplot2 = df_op.boxplot(rot=45,fontsize=5)
    fig2 = boxplot2.get_figure()
    fig2.savefig( "bp_count_opt_perm.pdf")

evaluate2(df1, df2)

from pandas boxplot contains content of plot saved before