title: R ggplot omit NA
body: <p>I'm trying to do ggplots in R. My dataset has a few rows with NA for one or more variables. How do I get the NA not to show up on the graph?
  Here's my code:</p>
  
  <pre><code>met$marker_Degree2 = factor(met$marker_Degree, levels=c("none", "weak"))
  p4 &lt;- ggplot(met, aes(factor(marker_Degree2), avgtsh))
  p4 + ggtitle('Serum marker and Tumor marker') +
  geom_point(shape=21, size=4, aes(color=factor(marker_Degree2))) +
  scale_color_manual(values = c("orange", "green")) +
  theme_bw() +
  xlab("Marker Tissue Staining Degree") +
  ylab("Mean marker Level in Serum")
  </code></pre>