title: R ggplot with two series: points and errorbars with legends
body: <p>If I have a dataframe like this:</p>
  
  <pre><code>obs&lt;-rnorm(20)
  d&lt;-data.frame(year=2000:2019,obs=obs,pred=obs+rnorm(20,.1))
  d$pup&lt;-d$pred+.5
  d$plow&lt;-d$pred-.5
  d$obs[20]&lt;-NA
  d
  </code></pre>
  
  <p>And I want the observation and model prediction error bars to look something like:</p>
  
  <pre><code>(p1&lt;-ggplot(data=d)+aes(x=year)
   +geom_point(aes(y=obs),color='red',shape=19)
   +geom_point(aes(y=pred),color='blue',shape=3)
   +geom_errorbar(aes(ymin=plow,ymax=pup))
   )
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/hLXL4.png" alt="observations and predictive model error bars"></p>
  
  <p>How do I add a legend/scale/key identifying the red points as observations and the blue plusses with error bars as point predictions with ranges?</p>