title: R: Melting an data frame and plotting by group
body: <p>I have a large dataset that I'd like to use to plot genetic divergence along chromosomes. The data frame I am using has the following format. </p>
  
  <pre><code>ID      Group   100     270     310     430     460     550     580     660     710     740
  Strain1 A       0.191   0.147   0.124   0.149   0.193   0.189   0.123   0.189   0.151   0.180
  Strain2 A       0.188   0.188   0.149   0.136   0.000   0.199   0.199   0.188   0.149   0.000
  Strain3 B       0.123   0.147   0.190   0.061   0.148   0.149   0.148   0.197   0.178   0.172
  Strain4 B       0.147   0.197   0.188   0.178   0.179   0.149   0.191   0.154   0.179   0.187
  </code></pre>
  
  <p>I'd like to use ggplot2 to plot a line for each strain, with the lines colored according to group affiliation, and a continuous x-axis running from chromosome positions 100 through 740. I cannot figure out how to melt the data without extracting the group info first and then adding it back after melting. Can anyone suggest a one-step approach to accomplish this?</p>