title: How to map a continuous variable on a plot as an extra layer?
body: <p>I'm trying to map a continuous variable on my plot. Perhaps by means of color gradient. But I have not succeeded yet. Any suggestion for adding "gamma" to my plot? I would like also to indicate location of each category (2 levels) on the plot, is that feasible on a single plot?</p>
  
  <pre><code> plot_2 &lt;- ggplot(data=states_2)+  ggtitle("TBD ")+
   xlab("q")+ylab("r")+ geom_point(aes(x=Q , y= Cor, color=metric))+geom_smooth (aes(x=Q, y=Cor, 
  color=metric,group=(metric)),
   method="lm", se=FALSE)
  
     Cor  metric  K     Q  Category     gamma
    0.33  APD    2s   0.4mu   64spp   -0.25282382
    0.23  APD    2s   0.4mu   64spp   -0.653438937
    0.21  APD    2s   0.4mu   64spp   0.799639202
    0.14  APD    2s   0.4mu   64spp   1.039215902
    0.37  APD    2s   0.4mu   64spp   0.207669854
  </code></pre>