title: ggplot object not found error when adding layer with different data
body: <p>I have a plot with some points and I'd like to use segment to connect them</p>
  
  <pre><code>dummy = data.frame(GROUP=c("A","B","C","D"),
                     X = c(80,75,68,78),
                     Y=c(30, 32,36,33)
  
  )
  df= data.frame(x1 = c(80), x2 =c(78) , y1=c(30), y2 =c(33))
  df
  library(ggplot2)
  ggplot(dummy,aes(x=X,y=Y,color=GROUP)) + 
    geom_point() +
    geom_segment(aes(x=x1,y=y1,xend= x2, yend =y2), data = df) 
  </code></pre>
  
  <p>but I get this error </p>
  
  <pre><code>Error in eval(expr, envir, enclos) : object 'GROUP' not found
  </code></pre>
  
  <p>What am I doing wrong here?</p>