title: merging several data frames into a single expanded frame
body: <p>I have a list of data frames, where each frame contains the same kind of measurements for a single system. E.g.,</p>
  
  <pre><code>$system1                           
                  file    cumSize     cumloadTime     query1
  1  ../data/data1.dat    100000      158.1000        0.4333333
  2  ../data/data2.dat    200000      394.9000        0.5000000
  3  ../data/data3.dat    250000      561.8667        0.6666667
  
  $system2                           
                  file    cumSize     cumloadTime     query1
  1  ../data/data1.dat    100000      120.1000        0.4333333
  2  ../data/data2.dat    200000      244.9000        0.4500000
  3  ../data/data3.dat    250000      261.8667        0.2666667
  </code></pre>
  
  <p>Now I would like to display several aspects of these data frames in separate plots using the <strong>matplot</strong> command. Therefore I need to transform the above input data structure into the following output structure:</p>
  
  <pre><code>$cumloadTime
  
  cumSize     system1     system2
  100000      158.1000    120.1000
  200000      394.9000    244.9000
  250000      561.8667    261.8667
  
  $query1
  
  cumSize     system1     system2
  100000      0.4333333   0.4333333
  200000      0.5000000   0.4500000
  250000      0.6666667   0.2666667
  </code></pre>
  
  <p>I played around with the <strong>reshape</strong>, <strong>merge</strong>, and <strong>melt</strong> functions but haven't found the solution yet.</p>
  
  <p>Thanks for any hints...</p>