title: How to plot additional points on the top of scatter plot?
body: <p>I have panda dataframe as df with two attributes df.one (=x) and df.two (=y). Now, I want to plot scatter plot for these data points. I used </p>
  
  <pre><code>ax1 = fig.add_subplot(111)
  ax1.scatter(df.one,df.two,c = 'g',marker = 'o',alpha = 0.2)
  </code></pre>
  
  <p>Now, I want to plot centroid of the data points give by C. How should I overlay centroid on the above scatter plot? 
  I tried: </p>
  
  <pre><code>ax1.scatter(C[:,0],C[:,1],c = 'r',marker = 'x')
  </code></pre>
  
  <p>But it overrides the scatter plot, I want to overlay on that. Is there any hold on option, similar to <code>matlab</code>? </p>