title: Custom axis tick labels in ggplot2
body: <p>I would like to show a short time series showing heterogeneity of heroin seizures in Europe over the span of 22 years. However there are different amount countries included in some of the years. I would like to display this in the graph by putting "n=xx" for each year on the x-axis. Does anyone know how I should do this?
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  <pre><code>across_time&lt;- ggplot(by_year, aes(year, value) + 
            geom_errorbar(aes(ymin=value-se, ymax=value+se), width=.4) +
            geom_line(colour="black", size= 2) +
            geom_point(size=4, shape=21, fill="white") +  # 21 is filled circle
            xlab("Year") +
            ylab("Siezures") +  
            ggtitle("Hetrogeniety Across Time") +
            scale_x_continuous(breaks = round(seq(min(1990), max(2012), by=2)))
  across_time
  </code></pre>
  
  <p>Here is a link to what the graph looks like:</p>
  
  <p><a href="http://imgur.com/XWhBqqi" rel="nofollow">http://imgur.com/XWhBqqi</a></p>