title: How to split a data frame by rows, and then process the blocks?
body: <p>I have a data frame with several columns, one of which is a factor called "site". How can I split the data frame into blocks of rows each with a unique value of "site", and then process each block with a function? The data look like this:</p>
  
  <pre><code>site year peak
  ALBEN 5 101529.6
  ALBEN 10 117483.4
  ALBEN 20 132960.9
  ALBEN 50 153251.2
  ALBEN 100 168647.8
  ALBEN 200 184153.6
  ALBEN 500 204866.5
  ALDER 5 6561.3
  ALDER 10 7897.1
  ALDER 20 9208.1
  ALDER 50 10949.3
  ALDER 100 12287.6
  ALDER 200 13650.2
  ALDER 500 15493.6
  AMERI 5 43656.5
  AMERI 10 51475.3
  AMERI 20 58854.4
  AMERI 50 68233.3
  AMERI 100 75135.9
  AMERI 200 81908.3
  </code></pre>
  
  <p>and I want to create a plot of <code>year</code> vs <code>peak</code> for each site.</p>