title: How do I separate a data set on a scatter plot
body: <p>I'm very new to python but am interested in learning a new technique whereby I can identify different data points in a scatter plot with different markers according to where they fall in the scatter plot.</p>
  
  <p>My specific example is much to this: <a href="http://www.astroml.org/examples/datasets/plot_sdss_line_ratios.html" rel="nofollow">http://www.astroml.org/examples/datasets/plot_sdss_line_ratios.html</a></p>
  
  <p>I have a BPT plot and want to split the data along the demarcation line line.</p>
  
  <p>I have a data set in this format:</p>
  
  <pre><code>data = [[a,b,c],
          [a,b,c],
          [a,b,c]
  ]
  </code></pre>
  
  <p>And I also have the following for the demarcation line:</p>
  
  <pre><code>NII   = np.linspace(-3.0, 0.35)
  
  def log_OIII_Hb_NII(log_NII_Ha, eps=0):
      return 1.19 + eps + 0.61 / (log_NII_Ha - eps - 0.47)
  </code></pre>
  
  <p>Any help would be great!</p>