title: Subtract a column in pandas dataframe by its first value
body: <p>I need to subtract all elements in one column of pandas dataframe by its first value.</p>
  
  <p>In this code, pandas complains about self.inferred_type, which I guess is the circular referencing.</p>
  
  <pre><code>df.Time = df.Time - df.Time[0]
  </code></pre>
  
  <p>And in this code, pandas complains about setting value on copies.</p>
  
  <pre><code>df.Time = df.Time - df.iat[0,0]
  </code></pre>
  
  <p>What is the correct way to do this computation in Pandas?</p>