title: ggplot2: adjusting the number of points on a line graph
body: <p>I would like to lower the number of points on the lines on my plot.</p>
  
  <p>For example,</p>
  
  <pre><code>date &lt;- c("2017-04-15","2017-04-16","2017-04-17","2017-04-18","2017-04-19","2017-04-20","2017-04-21")
  x &lt;- c(1,3,3,4,3,5,2)
  
  df &lt;- data.frame(date,x)
  </code></pre>
  
  <p>Rather than having a point located at every vertex. I would like one located at every other vertex. The first, third, fifth and seventh vertex would have points while the others would not.</p>
  
  <pre><code>ggplot(df, aes(date,x,group=1)) +
    geom_line(size=.4) + 
    geom_point(size=.7)
  </code></pre>
  
  <p>This seems simple enough, but I have been unable to find any information on how to do it.</p>