title: How to plot only few series from a dataframe with ggplot2?
body: <p>I have a dataframe with multiple series and I would like to plot only few of them with ggplot2. Do you know how to do this easily wihtin ggplot 2 ?</p>
  
  <pre><code>tdat=structure(list(uL = c(6, 4, 2, 1, 0.5, 25, 20, 15, 6, 4, 2, 1, 
  0.5, 6, 4, 2, 1, 0.5, 25, 20, 15, 10, 25, 20, 15, 10, 5, 1), 
  Intensity = c(29355, 36034, 42571, 62325, 79505, 25190, 19721.5, 
  13144.5, 40385.5, 40507.5, 46156, 73707, 93496.75, 46940.75, 
  45588, 56215, 81908.5, 85508, 93243.5, 79260, 59858.5, 31522.5, 
  81629.4, 45928.5, 37694.5, 39466, 45914, 87105), sample = structure(c(3L, 
  3L, 3L, 3L, 3L, 1L, 1L, 1L, 4L, 4L, 4L, 4L, 4L, 2L, 2L, 2L, 
  2L, 2L, 6L, 6L, 6L, 6L, 5L, 5L, 5L, 5L, 5L, 5L), .Label = c("MCH Mild", 
  "PCAM MCH HIGH1", "PCAM MCH LOW-atp,E1E2,UbK48", "PCAM MCH LOW1", 
  "PCAM MCH LOW2", "RIPA MCH"), class = "factor")), .Names = c("uL", 
  "Intensity", "sample"), class = "data.frame", row.names = c(NA, 
  -28L))
  
  head(tdat)
      uL Intensity                      sample
  1  6.0     29355 PCAM MCH LOW-atp,E1E2,UbK48
  2  4.0     36034 PCAM MCH LOW-atp,E1E2,UbK48
  3  2.0     42571 PCAM MCH LOW-atp,E1E2,UbK48
  4  1.0     62325 PCAM MCH LOW-atp,E1E2,UbK48
  5  0.5     79505 PCAM MCH LOW-atp,E1E2,UbK48
  6 25.0     25190                    MCH Mild
  </code></pre>
  
  <p>I can plot this using :</p>
  
  <pre><code>p &lt;- ggplot(tdat, aes(uL, Intensity)) + geom_point(aes(colour = sample,shape=sample), size=5) + geom_smooth(level=0);p
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/VvUKA.jpg" alt="enter image description here"></p>
  
  <p>But I would like to plot only 3 series juste like that : </p>
  
  <pre><code>tdat=tdat[grepl("(PCAM MCH LOW1|PCAM MCH LOW-atp,E1E2,UbK48|PCAM MCH HIGH1)",tdat$sample),]
  p &lt;- ggplot(tdat, aes(uL, Intensity)) + geom_point(aes(colour = sample,shape=sample), size=5) + geom_smooth(level=0);p
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/lDX81.jpg" alt="enter image description here"></p>
  
  <p>Is there a quick way to do that without using grepl() ?</p>