title: numpy polyfit passing through 0
body: <p>Suppose I have <code>x</code> and <code>y</code> vectors with a weight vector <code>wgt</code>. I can fit a cubic curve (<code>y = a x^3 + b x^2 + c x + d</code>) by using <code>np.polyfit</code> as follows:</p>
  
  <pre><code>y_fit = np.polyfit(x, y, deg=3, w=wgt)
  </code></pre>
  
  <p>Now, suppose I want to do another fit, but this time, I want the fit to pass through <code>0</code> (i.e. <code>y = a x^3 + b x^2 + c x</code>, <code>d = 0</code>), how can I specify a particular coefficient (i.e. <code>d</code> in this case) to be zero?</p>
  
  <p>Thanks</p>