title: Jittering the lines in ggplot produces two symbols for one data point
body: <p>I am trying to create a simple line plot and jitter the lines.</p>
  <pre><code>library(ggplot2)
  test=read.table(text= &quot;
    group  x  y
    1 1   17  
    1 2   16  
    2 1   13
    2 2   12.5
    &quot;, header=TRUE, sep=&quot;&quot;  )        
  
  pd &lt;- position_dodge(.2)
  
  qplot(x=x, y=y, data=test, group=group, colour=group ) +
  geom_line(position=pd )+
  geom_point(position=pd, size=6)
  </code></pre>
  <p>I am getting</p>
  <pre><code>ymax not defined: adjusting position using y instead
  ymax not defined: adjusting position using y instead
  </code></pre>
  <p>warnings and somehow two symbols are displayed for each data point instead of one:
  <img src="https://i.stack.imgur.com/n1NXl.jpg" alt="enter image description here" /></p>
  <p>I assume the ymax warning has something to do with getting two symbols. I'd appreciate any help. thanks!</p>