title: adding x and y axis labels in ggplot2
body: <p>How do I change the x and y labels on this graph please?</p>
  
  <pre><code>library(Sleuth2)
  library(ggplot2)
  discharge&lt;-ex1221new$Discharge
  area&lt;-ex1221new$Area
  nitrogen&lt;-ex1221new$NO3
  p &lt;- ggplot(ex1221new, aes(discharge, area), main="Point")
  p + geom_point(aes(size= nitrogen)) + 
      scale_area() + 
      opts(title = expression("Weighted Scatterplot of Watershed Area vs. Discharge and Nitrogen Levels (PPM)"), 
           subtitle="n=41")
  </code></pre>