title: colour="black" in geom_smooth changes lm line with R gglplot2. Why?
body: <p>My examples are made with easy available data:</p>
  
  <pre><code>data(Salaries, package="car")
  library(tidyverse)
  </code></pre>
  
  <p>When running:</p>
  
  <pre><code>ggplot(Salaries, aes(x=yrs.since.phd, y=salary, color=rank))+
  geom_point() +
  geom_smooth(method="lm", size=0.5)+
  facet_grid(~sex)
  </code></pre>
  
  <p>I got this graph which creates an lm line for every rank:</p>
  
  <p><a href="https://i.stack.imgur.com/k82iu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/k82iu.png" alt="enter image description here"></a></p>
  
  <p>But when I use</p>
  
  <pre><code>ggplot(Salaries, aes(x=yrs.since.phd, y=salary, color=rank))+
  geom_point() +
  geom_smooth(method="lm", colour="black", size=0.5)+
  facet_grid(~sex)
  </code></pre>
  
  <p>The graph shows now an unique lm line for all data:</p>
  
  <p><a href="https://i.stack.imgur.com/4lHqB.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/4lHqB.png" alt="enter image description here"></a></p>
  
  <p>Any idea of what's happening? Why setting colour="black" changes the whole appearence of the lm line in the graphic?</p>