title: Discrete values in US map legend using plot_usmap
body: <p>I am plotting a group of "treatment" counties, using usmap::plot_usmap(), such that treatment=1 for the identified counties an 0 otherwise. I would like the legend to reflect the fact that the variable is discrete (0/1) rather than show a continuous scale of colors?
  My dataset has 2 variables: fips and treatment.</p>
  
  <p>My code is:</p>
  
  <pre><code>library(usmap)
  library(ggplot2)
  
    plot_usmap(regions = "county", data=data, values = "treatment",color="grey")+ 
    theme(panel.background = element_rect(colour = "black"))+
    scale_fill_gradient(low = "white", high = "blue",name = "treatment",
                        breaks=c(0,1), limits = c(0, 1)) + 
    theme(legend.position = "right") 
  </code></pre>
  
  <p>The output graph is:
  <a href="https://i.stack.imgur.com/svaVO.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/svaVO.png" alt="enter image description here"></a></p>