title: Replace each value in Series with its relative ranking
body: <p>I have a sorted Series, is there a simple way to change it from</p>
  <pre><code>A    0.064467
  B    0.042283
  C    0.037581
  D    0.017410
  dtype: float64
  </code></pre>
  <p>to</p>
  <pre><code>A    1
  B    2
  C    3
  D    4
  </code></pre>