title: What does Continuous x aesthetic -- did you forget aes(group=...) mean?
body: <pre><code>Scatterplot&lt;-ggplot(
       diamonds[sample(nrow(diamonds), 1000), ],
       aes(carat, price, colour=clarity)
    )+
    geom_point(position="jitter", alpha=0.6)+
    facet_grid(~cut) + 
    scale_x_continuous(breaks=seq(0, 5, 1)) + 
    geom_boxplot(alpha=0, colour="black") +  
    scale_color_brewer(palette = "Set1")
    plot(Scatterplot)
  </code></pre>
  
  <p>I cannot find any clue about this warning. Anyone knows? Please check the picture:</p>
  
  <p><a href="https://i.stack.imgur.com/LDXT4.png" rel="noreferrer"><img src="https://i.stack.imgur.com/LDXT4.png" alt="output"></a></p>