title: R - ggplot line color (using geom_line) doesn't change
body: <p>When ploting 2 lines on one plot with ggplot(geom_line) the color of the line doesn't compre to the color that I set. I want the lines black and blue, but the outcome is red and blue. I tried it without (first code) and with (second) 'scale_color_manual', also tried with colour insted of color, with the same result:</p>
  
  <p>fisrt code:</p>
  
  <pre><code>  ggplot(data=main_data) +
    # black plot
    geom_line(aes(x=vectors_growth_rate_with_predator, 
                  y=disease_prevalnce_with_predator,
                  color = "black")) + 
    # blue plot
    geom_line(aes(x=vectors_growth_rate_with_predator, 
                  y=disease_prevalnce_without_predator,
                  color = "blue"))
  </code></pre>
  
  <p>second code: </p>
  
  <pre><code>PrevVSGrowth = 
    ggplot(data=main_data) +
    # black plot
    geom_line(aes(x=vectors_growth_rate_with_predator, 
                  y=disease_prevalnce_with_predator)) + 
    # blue plot
    geom_line(aes(x=vectors_growth_rate_with_predator, 
                  y=disease_prevalnce_without_predator))
  
  PrevVSGrowth + scale_color_manual(values=c(disease_prevalnce_with_predator= 'black',
                                             disease_prevalnce_without_predator = 'blue'))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/garD0.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/garD0.jpg" alt="enter image description here"></a></p>