title: Create different colours for regression lines
body: <p>I am creating a scatterplot (around 1,500,000 points), and I running regressions through it based on a factor variable (see below "halfs"). This is the picture:</p>
  
  <p><img src="https://i.stack.imgur.com/3tSvs.png" alt="enter image description here"> </p>
  
  <p>As you can see it is difficult to see the "red" regression lines.</p>
  
  <p>This is the data:</p>
  
  <pre><code>  For_Cov     Per_chg      halfs
    1  0.8372001 0.002400000 upper half
    2  0.7236001 0.002800111 upper half
    3  0.6036000 0.000800000 upper half
    4  0.8540000 0.000000000 upper half
    5  0.9080001 0.003200000 upper half
    6  0.8248000 0.000000000 upper half
    7  0.1132000 0.000000000 upper half
    8  0.2044000 0.007600000 upper half
    9  0.2476001 0.085200000 upper half
    10 0.2368000 0.003600000 upper half
  </code></pre>
  
  <p>This is the code:</p>
  
  <pre><code> ggplot(grid_gdp_full, aes(x = For_Cov, y = Def_Chg, group = factor(halfs))) + 
      geom_point(aes(colour = halfs), alpha = 0.1) +
      stat_smooth(method = "lm", formula = y ~ x + I(x^2), size = 4, fullrange=TRUE,     aes(group = halfs, colour = halfs), alpha = 1) +
      xlab("Initial") +
      ylab("Percent") +
      ylim(0,0.10) +
      scale_x_reverse() +
      theme_bw() +theme(
      axis.text=element_text(size=12)
      ,axis.title=element_text(size=14,face="bold")
      ,plot.background = element_blank()
      ,panel.grid.major = element_blank()
      ,panel.grid.minor = element_blank()
      ,panel.border = element_blank()
      ,panel.background = element_blank()
       ) +
       theme(axis.line = element_line(color = 'black')) 
  </code></pre>
  
  <p>Does anyone how to change the code so that the regression lines will have different colors (regression line is created by stat_smooth) - possibly grey and blue for the two factor levels without changing the initial color of the points?</p>
  
  <p>Here is the code I am using at the moment and I am still not able to get the two different colored lines:</p>
  
  <pre><code> ggplot(grid_gdp_full, aes(x = For_Cov, y = Def_Chg, fill = factor(halfs))) + 
     geom_point(aes(colour = halfs), alpha = 0.1, colour="transparent",shape=21) +
     stat_smooth(method = "lm", formula = y ~ x + I(x^2), size = 2, fullrange=TRUE, aes(group = factor(halfs)), alpha = 1) +
     xlab("Initial") +
     ylab("Percent") +
     ylim(0,0.1) +
     scale_x_reverse() +
     theme_bw() +theme(
       axis.text=element_text(size=12)
       ,axis.title=element_text(size=14,face="bold")
       ,plot.background = element_blank()
       ,panel.grid.major = element_blank()
       ,panel.grid.minor = element_blank()
       ,panel.border = element_blank()
       ,panel.background = element_blank()
     ) +
     theme(axis.line = element_line(color = 'black')) +
     scale_colour_manual(values = c("red","blue")) + 
     scale_fill_manual(values = c("grey","green"))
  
  dev.off()
  </code></pre>