title: Using geom_line with multiple groupings
body: <p>I have a table as follows:</p>
  
  <pre><code>&gt; testsizes
      size value replicate lane
  361   16  6898         1   L1
  362   17 10707         1   L1
  363   18  1786         1   L1
  364   19  1721         1   L1
  365   20  2454         1   L1
  421   16  8486         2   L1
  422   17 26691         2   L1
  423   18  3241         2   L1
  424   19  5068         2   L1
  425   20  7579         2   L1
  481   16  4152         3   L1
  482   17  4452         3   L1
  483   18   899         3   L1
  484   19  1973         3   L1
  485   20  2595         3   L1
  571   16  8284         1   L2
  572   17  9045         1   L2
  573   18  5041         1   L2
  574   19  7160         1   L2
  575   20  9730         1   L2
  631   16  5639         2   L2
  632   17  9773         2   L2
  633   18  2433         2   L2
  634   19  3017         2   L2
  635   20  3864         2   L2
  691   16 10161         3   L2
  692   17 18609         3   L2
  693   18  3760         3   L2
  694   19  3543         3   L2
  695   20  4257         3   L2
  
  &gt; dput(testsizes)
  structure(list(size = c(16L, 17L, 18L, 19L, 20L, 16L, 17L, 18L, 
  19L, 20L, 16L, 17L, 18L, 19L, 20L, 16L, 17L, 18L, 19L, 20L, 16L, 
  17L, 18L, 19L, 20L, 16L, 17L, 18L, 19L, 20L), value = c(6898L, 
  10707L, 1786L, 1721L, 2454L, 8486L, 26691L, 3241L, 5068L, 7579L, 
  4152L, 4452L, 899L, 1973L, 2595L, 8284L, 9045L, 5041L, 7160L, 
  9730L, 5639L, 9773L, 2433L, 3017L, 3864L, 10161L, 18609L, 3760L, 
  3543L, 4257L), replicate = c("1", "1", "1", "1", "1", "2", "2", 
  "2", "2", "2", "3", "3", "3", "3", "3", "1", "1", "1", "1", "1", 
  "2", "2", "2", "2", "2", "3", "3", "3", "3", "3"), lane = c("L1", 
  "L1", "L1", "L1", "L1", "L1", "L1", "L1", "L1", "L1", "L1", "L1", 
  "L1", "L1", "L1", "L2", "L2", "L2", "L2", "L2", "L2", "L2", "L2", 
  "L2", "L2", "L2", "L2", "L2", "L2", "L2")), .Names = c("size", 
  "value", "replicate", "lane"), row.names = c(361L, 362L, 363L, 
  364L, 365L, 421L, 422L, 423L, 424L, 425L, 481L, 482L, 483L, 484L, 
  485L, 571L, 572L, 573L, 574L, 575L, 631L, 632L, 633L, 634L, 635L, 
  691L, 692L, 693L, 694L, 695L), class = "data.frame")
  </code></pre>
  
  <p>I want to make a line plot using ggplot that shows the change in value across sizes. At the moment I have this, amongst the other combinations I have tried:</p>
  
  <pre><code>ggplot(testlengths, aes(size, value, group=replicate, colour=replicate)) 
      + geom_line()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/I4kPm.png" alt="enter image description here"></p>
  
  <p>It looks like its trying to incorporate both lanes into the same series. But I can't find a way to set the lanes as another factor to group on. I want the lines to be grouped based on both the replicate and lane categories. The lanes should be coloured but the replicates do not need to be distinguished between each other.</p>
  
  <p>I am aware that I can probably achieve this by concatenating the two groups into one group beforehand. However, before I go down that route, I am wandering if ggplot can group by more than one grouping in a line plot without facets (I need to use facets later for another grouping)? I feel like it should be able to.</p>