title: geom_vline not showing up
body: <p>I am doing a scatterplot with a vertical line as intercept over a x-axis date variable, and the vertical line doesn't show up.</p>
  
  <p>I have tried with and without converting the intercept to number.</p>
  
  <pre><code>dis &lt;- read.csv("dis.csv")
  
  selection.districts &lt;- dis[which(dis$district=="Kien Svay"),]
  
  ggplot(selection.districts, aes(x=date, y=radiance)) + 
  geom_point() +
  geom_vline(aes(xintercept=as.numeric(intercept))) +
  geom_smooth(method="auto") +
  labs(x="Date", y = "Radiance")
  </code></pre>
  
  <p>I expect to see a vertical line based on the value of the intercept variable of the dataset. You can find a data sample to reproduce it here:</p>
  
  <p><a href="https://raw.githubusercontent.com/nreguera/NightLights/master/dis.csv" rel="nofollow noreferrer">https://raw.githubusercontent.com/nreguera/NightLights/master/dis.csv</a></p>
  
  <p>Thanks.</p>