title: ggplot with missing values
body: <p>I have a dataset like this</p>
  
  <pre><code>data &lt;- data.frame(Time=as.Date(c("2007-01-31", "2007-02-28", "2007-03-31", "2007-04-30", 
          "2007-05-31", "2007-06-30", "2007-07-31", "2007-08-31", "2007-09-30", "2007-10-31", 
          "2007-11-30", "2007-12-31"), format="%Y-%m-%d"), 
          a=c(NA, NA, 1.201, NA, NA, 1.206, NA, NA, 1.212, NA, NA, 1.237), 
          b=c(1.187, 1.201, 1.206, 1.219, 1.211, 1.187, 1.202, 1.213, 1.209, 1.208, 
            1.219, 1.237), 
          c=c(1.198, 1.201, 1.203, 1.206, 1.207,1.207, 1.208, 1.21, 1.214, 1.22, 1.228, 1.236))
  &gt; data
       Time     a     b     c
  1  2007-01-31    NA 1.187 1.198
  2  2007-02-28    NA 1.201 1.201
  3  2007-03-31 1.201 1.206 1.203
  4  2007-04-30    NA 1.219 1.206
  5  2007-05-31    NA 1.211 1.207
  6  2007-06-30 1.206 1.187 1.207
  7  2007-07-31    NA 1.202 1.208
  8  2007-08-31    NA 1.213 1.210
  9  2007-09-30 1.207 1.209 1.214
  10 2007-10-31    NA 1.208 1.220
  11 2007-11-30    NA 1.219 1.228
  12 2007-12-31 1.219 1.237 1.236
  </code></pre>
  
  <p>I want a ggplot with points for a and lines for b &amp; c. I came up with</p>
  
  <pre><code>ggplot(data, aes(x=Time)) +  geom_point(aes(y=a, colour="a"), size=3) + 
    geom_line(aes(y=b, colour="b")) + geom_line(aes(y=c, colour="c"))
  </code></pre>
  
  <p>The problem is, that I have many more variables like b &amp; c, which I want to plot. Is there a more efficient way to do it?</p>