title: Applying a continuous scale while using geom_label_repel based off separate dataframe
body: <p>Based off the approach outline in the answer <a href="https://stackoverflow.com/questions/50026999/how-to-add-a-single-label-to-a-stacked-bar-chart-in-ggplot2">here</a> I have a plot which draws it labels off a separate dataframe. However I was wondering if there would be a way to use <code>geom_label_repel</code> instead of <code>geom_text</code> but have a continuous colour scale applied to the label? </p>
  
  <p>So the original plot would be: </p>
  
  <pre><code>df %&gt;% 
   ggplot(aes(variable, value, width =.7)) +
   geom_col(col="black",aes(fill=X1)) + #Black border around the bars
   expand_limits(y = 0) +
   scale_y_continuous(limits = c(0,70), 
                      breaks=c(0,10,20, 30,40, 50,60,70), 
                      expand = c(0.02, 0.02))+
   geom_label_repel(data = df_max,
   aes(label = max, y = max))+ #Here the text is added
   scale_fill_brewer(palette="Dark2") + #Changes the colour scale
   labs(x="Breed",
        y="Weighted score",
        fill="") +
   theme_bw() +
   theme(panel.grid.major = element_blank(),
         panel.grid.minor = element_blank(),
         legend.direction="vertical", legend.position=c(0.9, 0.85), 
         legend.text=element_text(size=11),
         legend.key = element_rect(size = 4),
         legend.key.size = unit(1.5, 'lines'))
  </code></pre>
  
  <p>My guess, if it's possible, its going to include a variation of <code>scale_fill_gradient</code> but I haven't found a way to make it work?</p>
  
  <p>If I include a <code>scale_fill_gradient</code> like this:</p>
  
  <pre><code> df %&gt;% 
    ggplot(aes(variable, value, width =.7)) +
    geom_col(col="black",aes(fill=X1)) + #Black border around the bars
    expand_limits(y = 0) +
    scale_fill_gradient2(data=df_max, aes(y=max), 
                         fill=max,
                         low='blue', mid='green', high='red', midpoint = 10)+
    scale_y_continuous(limits = c(0,70), 
                       breaks=c(0,10,20, 30,40, 50,60, 70),         
                       expand = c(0.02, 0.02))+
    geom_label_repel(data = df_max,
                     aes(label = max, y = max))+ #Here the text is added
    scale_fill_brewer(palette="Dark2") + #Changes the colour scale
    labs(x="Breed",
         y="Weighted score",
         fill="") +
    theme_bw() +
    theme(panel.grid.major = element_blank(),
          panel.grid.minor = element_blank(),
          legend.direction="vertical", 
          legend.position=c(0.9, 0.85), 
          legend.text=element_text(size=11),
          legend.key = element_rect(size = 4),
          legend.key.size = unit(1.5, 'lines'))  # Ensures spacing between the 
                                                 # different lines on the legend.
  </code></pre>
  
  <p>I get the following error:</p>
  
  <blockquote>
  <pre><code>Error in continuous_scale("fill", "gradient2", div_gradient_pal(low, mid,  : 
    unused arguments (data = list(variable = 1:4, max = c(15, 14.7, 9.7, 4.1)), 
    fill = .Primitive("max"))
  </code></pre>
  </blockquote>