title: scale_fill_manual is not working in geom_bar
body: <p>I'm trying to change the color in a barplot in ggplot2 using scale_fill_manual, but for some reason only works if i use the fill option inside aesthetics.
  I made an example:</p>
  
  <pre><code>library(ggplot2)
  library(dplyr) 
  iris %&gt;% ggplot(aes(x=Sepal.Width,y=Sepal.Length))+
  geom_bar(stat="identity") + scale_fill_manual(values='lightblue')
  </code></pre>
  
  <p>Here is the result, no changing in the color:
  <a href="https://i.stack.imgur.com/Ik81h.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Ik81h.png" alt="enter image description here"></a></p>
  
  <p>Now, using the fill option inside aesthetics, it works:</p>
  
  <pre><code>iris %&gt;% ggplot(aes(x=Sepal.Width,y=Sepal.Length, fill=factor(2) ))+
  geom_bar(stat="identity")+scale_fill_manual(values='lightblue')
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/YHRW2.png" rel="noreferrer"><img src="https://i.stack.imgur.com/YHRW2.png" alt="enter image description here"></a></p>
  
  <p>There is some way to change the bar colour without using the fill option, only using scale_fill_manual?</p>