title: Split data using timeframe into groups in python pandas
body: <p>I have a dataframe called df which like this but it is actually [9147 rows x 3 columns]</p>
  
  <pre><code>indexID  RngUni[m]  PowUni[dB]
  157203   1.292283      132
  157201   1.271878      132
  157016   1.285481      134
  157404   1.305886      136
  157500   1.353496      136
  157524   1.251474      136
  157227   1.292283      132
  157543   1.339893      136
  157903   1.353496      138
  156928   1.299084      134
  157373   1.299084      136
  156937   1.414709      134
  157461   1.353496      136
  157718   1.360297      138
  157815   1.326290      138
  157806   1.271878      134
  156899   1.360298      134
  157486   1.414709      138
  157628   1.271878      136
  157405   1.299084      134
  157244   1.299084      134
  157522   1.258275      136
  157515   1.367099      138
  157086   1.305886      136
  157602   1.251474      134
  157131   1.265077      132
  157170   1.380702      138
  156904   1.360297      134
  157209   1.401106      138
  157018   1.265077      134
  </code></pre>
  
  <p>What I am trying to do is to pick certain values of the data in the table.</p>
  
  <p><code>df.plot(x = 'RngUni[m]', y = 'PowUni[dB]', kind = 'scatter')</code> gives:<a href="https://i.stack.imgur.com/39V47.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/39V47.png" alt="enter image description here"></a></p>
  
  <p>Assuming that the main group is the area where most of the data points cluster, what I need to do is to pick 80% of the points that are in the main group and 20% of points that are outside the main group.</p>
  
  <p>I need the indexID of all the points outputted as a list. How can I do this?</p>
  
  <p>An example of the clustering required. What I would like to do is to pick 80% of the points in the circle and 20% of the points outside the circle.
  <a href="https://i.stack.imgur.com/xtjWZ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/xtjWZ.png" alt="enter image description here"></a></p>