title: How to find a row that in a given table return maximize return and minimalize variance
body: <p>Suppose I have a DataFrame that consists of three columns (index, return, volatility) and five rows.
  I would like to receive the index number that maximizes value of return AND minimizes value of volatility however the second condition is less important than first. </p>
  
  <p>How can I receive that? I tried to apply method .rank() for both columns and then sort by them but effect was poor.</p>