title: geom_rect() main variable not found
body: <p>I want to plot rectangle "shading" in with my ggplot. The ggplot code works and provides the image shown below. I looked for information <a href="https://stackoverflow.com/questions/9968975/using-ggplot2-in-r-how-do-i-make-the-background-of-a-graph-different-colours-in">here</a> and constructed a data frame for the x and y values.</p>
  
  <pre><code>mydf&lt;-data.frame(tiempo=df5$tiempo,vel=df5$TR2x45.17)[1:14,]
  structure(list(tiempo = c(618.2, 618.4, 618.6, 618.8, 619, 619.2, 
  619.4, 619.6, 619.8, 620, 620.2, 620.4, 620.6, 620.8), vel = c(0, 
  0, -4, -9, 5, 9, 1, 4, 0, 0, -1, -4, 0, 1)), .Names = c("tiempo", 
  "vel"), row.names = c(NA, 14L), class = "data.frame")
  
  rects &lt;- data.frame(xstart = seq(618,619.5,.5), xend = seq(618.5,620,.5), col = letters[1:4])
  
  ggplot(data=mydf,
         aes(x=tiempo,y=vel))+theme_minimal()+
             geom_point(size=4)+
      labs(title=c("Velocidad ejemplo pasaje figura"))+
         geom_smooth(method="loess", span=.3, se=FALSE, colour="red", size=1,alpha=0.5) +
  geom_rect(data = rects, aes(xmin = xstart, xmax = xend, ymin = -Inf, ymax = Inf, fill = col), alpha = 0.4) 
  </code></pre>
  
  <p>If I run the code until the <code>geom_smooth(...)</code> line produces the plot. If I add <code>geom_rect(...)</code> it gives this error back:
  <strong>Error in eval(expr, envir, enclos) : object 'tiempo' not found</strong></p>
  
  <p><img src="https://i.stack.imgur.com/8dXxJ.png" alt="enter image description here"></p>
  
  <p>I don't understand what does it mean that "tiempo" is not being found while everyother thing finds it. Also, I'm using another dataframe with <code>geom_rect()</code> so why is it looking for tiempo there?</p>