title: Change the symbol of specific data points in line plot, base R
body: <p>I want to change the symbol of particular data points on a line graph in base R.</p>
  
  <p>Here is an example of my plot:</p>
  
  <pre><code>month&lt;-c("2010-08-01", "2010-09-01", "2010-10-01", "2010-12-01", "2011-01-01", "2011-02-01",
       "2011-03-01", "2011-04-01", "2011-05-01", "2011-06-01", "2011-07-01", "2011-09-01",
       "2011-11-01", "2012-01-01", "2012-02-01", "2012-03-01", "2012-05-01", "2012-07-01",
       "2012-08-01")
  prevalence&lt;-c(10,7.5,5.2,3.5,6.4,2.7,5.8,13.2,4.3,4.7,6.4,4.4,5.2,3.3,1.0,3.1,9.9,33.3,1.0)
  df&lt;-data.frame(month, prevalence)
  df$month&lt;-as.Date(df$month)
  plot(df$month, df$prevalence,lwd = 1.8, ylim=c(0,40),pch=16, bty='n', 
   ylab="Prevalence (%)", xlab="Month",col='black',cex=1,cex.lab=1.0,cex.axis=1.0)
  len = .07
  axis(side = 1, at = df$month, labels=F, tck=-0.015)
  axis(side=2, at=c(0,10,20,30,40,50), labels=c("", "", "", "", "", ""), tck=-0.015)
  lines(df$month, df$prevalence, col='black', lwd=1.8)      
  </code></pre>
  
  <p>The graph produced:<img src="https://i.stack.imgur.com/vMXt6.png" alt="enter image description here"></p>
  
  <p>The y-value for the second last point is much higher than the others, and I would like to change the point to an open circle to indicate that there were only 3 samples for this data point. As I have a several of these line graphs to overlay in the one plot, it would look to messy to try to indicate this another way like manually overlaying a symbol on the graph.</p>