title: Adding legend to two geom_point()
body: <p>I just wondering is it possible to add legend, when we got two diffrent sets of points.</p>
  
  <pre><code>samochodziki %&gt;% 
    mutate(dopasowane = model_classic$fitted.values) %&gt;% 
    arrange(lp100k) %&gt;%
    mutate(index = 1:nrow(samochodziki)) %&gt;%
    ggplot(aes(x = index)) +
    geom_point(aes(y = lp100k), color = "red") +
    geom_point(aes(y = dopasowane), color = "blue")
  </code></pre>
  
  <p>Data on which I working:</p>
  
  <pre><code>structure(list(lp100k = c(9.8006076285327, 11.2006944326088, 
  7.4671296217392, 12.1244630456075, 13.0674768380436, 8.11084769257879
  ), cylinders = c(4L, 6L, 4L, 6L, 6L, 4L), displacement = c(1474.8354, 
  3277.412, 1458.44834, 3801.79792, 3687.0885, 1114.32008), horsepower = c(75L, 
  85L, 71L, 90L, 105L, 49L), weight = c(956.17271596, 1173.44346119, 
  902.6488163, 1456.0315077, 1415.66178677, 846.85695479), acceleration = c(15.5, 
  16, 14.9, 17.2, 16.5, 19.5), year = c(74L, 70L, 78L, 78L, 73L, 
  73L), origin = c(2L, 1L, 2L, 1L, 1L, 2L), name = c("fiat 128", 
  "ford maverick", "volkswagen scirocco", "amc concord", "plymouth valiant", 
  "fiat 128")), .Names = c("lp100k", "cylinders", "displacement", 
  "horsepower", "weight", "acceleration", "year", "origin", "name"
  ), row.names = c(NA, 6L), class = c("tbl_df", "tbl", "data.frame"
  ))
  </code></pre>
  
  <p>Model_classic shape</p>
  
  <pre><code>Call:
  lm(formula = lp100k ~ horsepower + weight + year, data = samochodziki)
  
  Residuals:
      Min      1Q  Median      3Q     Max 
  -3.3229 -0.7818 -0.0626  0.6344  6.3843 
  
  Coefficients:
                Estimate Std. Error t value Pr(&gt;|t|)    
  (Intercept) 22.8668314  1.8503686  12.358  &lt; 2e-16 ***
  horsepower   0.0151390  0.0042094   3.596 0.000375 ***
  weight       0.0068079  0.0003937  17.294  &lt; 2e-16 ***
  year        -0.2950063  0.0230468 -12.800  &lt; 2e-16 ***
  </code></pre>
  
  <p>So I want to add legend that for red points got "real", and for blue points "fitted".</p>