title: Pandas: Adding new column to dataframe which is a copy of the index column
body: <p>I have a dataframe which I want to plot with matplotlib, but the index column is the time and I cannot plot it.</p>
  
  <p>This is the dataframe (df3):</p>
  
  <p><a href="https://i.stack.imgur.com/BhfmS.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/BhfmS.jpg" alt="enter image description here"></a></p>
  
  <p>but when I try the following:</p>
  
  <pre><code>plt.plot(df3['magnetic_mag mean'], df3['YYYY-MO-DD HH-MI-SS_SSS'], label='FDI')
  </code></pre>
  
  <p>I'm getting an error obviously:</p>
  
  <pre><code>KeyError: 'YYYY-MO-DD HH-MI-SS_SSS'
  </code></pre>
  
  <p>So what I want to do is to add a new extra column to my dataframe (named 'Time) which is just a copy of the index column.</p>
  
  <p>How can I do it?</p>
  
  <p>This is the entire code:</p>
  
  <pre><code>#Importing the csv file into df
  df = pd.read_csv('university2.csv', sep=";", skiprows=1)
  
  #Changing datetime
  df['YYYY-MO-DD HH-MI-SS_SSS'] = pd.to_datetime(df['YYYY-MO-DD HH-MI-SS_SSS'], 
                                                 format='%Y-%m-%d %H:%M:%S:%f')
  
  #Set index from column
  df = df.set_index('YYYY-MO-DD HH-MI-SS_SSS')
  
  #Add Magnetic Magnitude Column
  df['magnetic_mag'] = np.sqrt(df['MAGNETIC FIELD X (μT)']**2 + df['MAGNETIC FIELD Y (μT)']**2 + df['MAGNETIC FIELD Z (μT)']**2)
  
  #Subtract Earth's Average Magnetic Field from 'magnetic_mag'
  df['magnetic_mag'] = df['magnetic_mag'] - 30
  
  #Copy interesting values
  df2 = df[[ 'ATMOSPHERIC PRESSURE (hPa)',
            'TEMPERATURE (C)', 'magnetic_mag']].copy()
  
  #Hourly Average and Standard Deviation for interesting values 
  df3 = df2.resample('H').agg(['mean','std'])
  df3.columns = [' '.join(col) for col in df3.columns]
  
  df3.reset_index()
  plt.plot(df3['magnetic_mag mean'], df3['YYYY-MO-DD HH-MI-SS_SSS'], label='FDI')  
  </code></pre>
  
  <p>Thank you !!</p>