title: How to resample starting from the first element in pandas?
body: <p>I am resampling the following table/data:</p>
  
  <pre><code>Timestamp  L_x   L_y    L_a     R_x     R_y     R_a
  2403950   621.3 461.3   313     623.3   461.8   260
  2403954   622.5 461.3   312     623.3   462.6   260
  2403958   623.1 461.5   311     623.4   464     261
  2403962   623.6 461.7   310     623.7   465.4   261
  2403966   623.8 461.5   309     623.9   466.1   261
  2403970   620.9 461.4   309     623.8   465.9   259
  2403974   621.7 461.1   308     623     464.8   258
  2403978   622.1 461.1   308     621.9   463.9   256
  2403982   622.5 461.5   308     621     463.4   255
  2403986   622.4 462.1   307     620.7   463.3   254
  </code></pre>
  
  <p>The table goes on and on like that.
  The timestamps are in milliseconds. I did the following to resample it into 100milliseconds bin time:</p>
  
  <ol>
  <li><p>I changed the timestamp index into a datetime format</p>
  
  <p><code>df.index = pd.to_datetime((df.index.values*1e6).astype(int))</code></p></li>
  <li><p>I resampled it in 100milliseconds:</p>
  
  <p><code>df = df.resample('100L')</code></p></li>
  </ol>
  
  <p>The resulting resampled data look like the following:</p>
  
  <pre><code>Timestamp  L_x   L_y    L_a     R_x     R_y     R_a
  2403900   621.3 461.3   313     623.3   461.8   260
  2404000   622.5 461.3   312     623.3   462.6   260
  2404100   623.1 461.5   311     623.4   464     261
  2404200   623.6 461.7   310     623.7   465.4   261
  2404300   623.8 461.5   309     623.9   466.1   261
  </code></pre>
  
  <p>As we can see the first bin time is 2403900, which is 50milliseconds behind the first timestamp index of the original table. But i wanted the bin time to start from the first timestamp index from the original table, which is 2403950. like the following:</p>
  
  <pre><code>Timestamp  L_x   L_y    L_a     R_x     R_y     R_a
  2403950   621.3 461.3   313     623.3   461.8   260
  2404050   622.5 461.3   312     623.3   462.6   260
  2404150   623.1 461.5   311     623.4   464     261
  2404250   623.6 461.7   310     623.7   465.4   261
  2404350   623.8 461.5   309     623.9   466.1   261
  </code></pre>