title: polyfit, TypeError: must be str, not float
body: <p>Code is as follows:</p>
  
  <pre><code>m, b = np.polyfit(df['Combined Length'], df['Mean Runtime'], 1)
  </code></pre>
  
  <p>The line produces the error: 'TypeError: must be str, not float'. Why would it want a string? If I input (2,3,4) and (4,6,8) it works fine. It does not work if the two pandas columns are converted to ndarrays either.</p>
  
  <p>The problem occurs with the initial conversion to arrays.</p>