title: Python setting the extracted Year-month in the Datetime format
body: <p>I have a df with index in datetime format. I want to extract year-month of each sample. This I will use later for coloring the samples in scatter plots.</p>
  <p>My code:</p>
  <pre><code>df.index = 
  DatetimeIndex(['2019-10-01 08:16:57', '2019-10-01 08:43:54',
                 '2019-10-01 08:45:24', '2019-10-01 08:49:42',
                 '2019-10-01 09:01:04', '2019-10-01 09:07:04',
                 '2019-10-01 09:07:40', '2019-10-01 09:07:46',
                 '2019-10-01 09:14:09', '2019-10-01 09:14:57',
                 '2020-11-13 14:22:58', '2020-11-13 14:23:01',
                 '2020-11-13 14:31:40', '2020-11-13 14:31:58',
                 '2020-11-13 14:32:49', '2020-11-13 14:33:13',
                 '2020-11-13 14:36:16', '2020-11-13 14:40:03',
                 '2020-11-13 15:01:24', '2020-11-14 09:57:21'],
                dtype='datetime64[ns]', name='timestamp', length=104676, freq=None) 
  df['year-month'] = df.index.strftime('%Y-%m')
  print(df['year-month'])
  Index(['2019-10', '2019-10', '2019-10', '2019-10', '2019-10', '2019-10',
         '2019-10', '2019-10', '2019-10', '2019-10',
         ...
         '2020-11', '2020-11', '2020-11', '2020-11', '2020-11', '2020-11',
         '2020-11', '2020-11', '2020-11', '2020-11'],
        dtype='object', name='timestamp', length=104676)
  </code></pre>
  <p>Extracted <code>year-month</code> is in object type, not datetime type as df.index. I want to plot a scatter plot for the df. I want to color the sample based on the year and month it belongs. Now when I set <code>plt.scatter(x,y,c=df['year-month'])</code> it raises an error. So, converting it to timestamp format could solve the scatter plot issue.</p>