title: Count the number of observations that occur per day
body: <p>I have a pandas dataframe indexed by time. I want to know the total number of observations (i.e. dataframe rows) that happen each day.</p>
  
  <p>Here is my dataframe:</p>
  
  <pre><code>import pandas as pd
  
  data = {'date': ['2014-05-01 18:47:05.069722', '2014-05-01 18:47:05.119994', '2014-05-02 18:47:05.178768', '2014-05-02 18:47:05.230071', '2014-05-02 18:47:05.230071', '2014-05-02 18:47:05.280592', '2014-05-03 18:47:05.332662', '2014-05-03 18:47:05.385109', '2014-05-04 18:47:05.436523', '2014-05-04 18:47:05.486877'], 
          'value': [1, 1, 1, 1, 1, 1, 1, 1, 1, 1]}
  df = pd.DataFrame(data, columns = ['date', 'value'])
  print(df)
  </code></pre>
  
  <p>What I want is a dataframe (or series) that looks like this:</p>
  
  <pre><code>   date             value
  0  2014-05-01       2
  1  2014-05-02       3
  2  2014-05-03       2
  3  2014-05-04       2
  </code></pre>
  
  <p>After reaching a bunch of StackOverflow questions, the closest I can get is:</p>
  
  <pre><code>df['date'].groupby(df.index.map(lambda t: t.day))
  </code></pre>
  
  <p>But that doesn't produce anything of use.</p>