title: How do I sum time series data by day in Python? resample.sum() has no effect
body: <p>I am new to Python. How do I sum data based on date and plot the result?</p>
  
  <p>I have a Series object with data like:</p>
  
  <pre><code>2017-11-03 07:30:00      NaN
  2017-11-03 09:18:00      NaN
  2017-11-03 10:00:00      NaN
  2017-11-03 11:08:00      NaN
  2017-11-03 14:39:00      NaN
  2017-11-03 14:53:00      NaN
  2017-11-03 15:00:00      NaN
  2017-11-03 16:00:00      NaN
  2017-11-03 17:03:00      NaN
  2017-11-03 17:42:00    800.0
  2017-11-04 07:27:00    600.0
  2017-11-04 10:10:00      NaN
  2017-11-04 11:48:00      NaN
  2017-11-04 12:58:00    500.0
  2017-11-04 13:40:00      NaN
  2017-11-04 15:15:00      NaN
  2017-11-04 16:21:00      NaN
  2017-11-04 17:37:00    500.0
  2017-11-04 21:37:00      NaN
  2017-11-05 03:00:00      NaN
  2017-11-05 06:30:00      NaN
  2017-11-05 07:19:00      NaN
  2017-11-05 08:31:00    200.0
  2017-11-05 09:31:00    500.0
  2017-11-05 12:03:00      NaN
  2017-11-05 12:25:00    200.0
  2017-11-05 13:11:00    500.0
  2017-11-05 16:31:00      NaN
  2017-11-05 19:00:00    500.0
  2017-11-06 08:08:00      NaN
  </code></pre>
  
  <p>I have the following code:</p>
  
  <pre><code># load packages
  import pandas as pd
  import matplotlib.pyplot as plt
  
  # import painkiller data
  df = pd.read_csv('/Users/user/Documents/health/PainOverTime.csv',delimiter=',')
  
  # plot bar graph of date and painkiller amount
  times = pd.to_datetime(df.loc[:,'Time'])
  
  ts = pd.Series(df.loc[:,'acetaminophen'].values, index = times,
                 name = 'Painkiller over Time')
  ts.plot()
  </code></pre>
  
  <p>This gives me the following line(?) graph:</p>
  
  <p><a href="https://i.stack.imgur.com/hfhbo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hfhbo.png" alt="line graph of raw data"></a></p>
  
  <p>It's a start; now I want to sum the doses by date. However, this code fails to effect any change: The resulting plot is the same. What is wrong?</p>
  
  <pre><code>ts.resample('D',closed='left', label='right').sum()
  ts.plot()
  </code></pre>
  
  <p>I have also tried <code>ts.resample('D').sum()</code>, <code>ts.resample('1d').sum()</code>, <code>ts.resample('1D').sum()</code>, but there is no change in the plot. </p>
  
  <p>Is <code>.resample</code> even the correct function? I understand resampling to be <em>sampling</em> from the data, e.g. randomly taking one point per day, whereas I want to sum each day's values.</p>
  
  <p>Namely, I'm hoping for some result (based on the above data) like:</p>
  
  <pre><code>2017-11-03 800
  2017-11-04 1600
  2017-11-05 1900
  2017-11-06 NaN
  </code></pre>