title: Linetype and Guide options for ggplot2 when using geom_smooth with continuous variables
body: <p>I am using ggplot2 and the geom_smooth functions to make predicted lines from a lm model. I am using continuous y and x functions in my ggplot code. </p>
  
  <p>I am trying to create a different line type for each line in the plot (which is the variable "location" ranging from -1,0, or 1), and I would like each location listed in the guide (where -1 = Location A, 0 = Location B, and 1 = Location C). A replicable excerpt of my code is below:</p>
  
  <pre><code>#test scores that range from 1 to 7
  score &lt;- c(6, 3, 5, 6, 7, 2, 4, 6, 3, 5, 4, 3, 3, 1, 3, 3, 3, 5, 2, 3, 2, 2, 7, 3, 7,     5, 4, 1, 3, 2, 7, 6, 6, 3, 6)
  #location of the school
  location &lt;- c(1,  0,  0,  0,  1,  1,  1, -1,  1, -1,  0, -1,  0,  1,  0,  0,  0, -1,     0, -1,  0, -1,  0,  0,  0,  0,  0, -1,  0, -1,  0,  0,  0,  1,  0)
  
  IQ &lt;- c(0.7171425604,    0.7056850461,    1.3929736220,    0.0633936624,   -0.6872828336,   -1.3665840767,    1.4368569944, 0.7297599487,   -0.5735047485,   -0.6752912747,   -0.6213572428,   -0.6110533924,   -0.7090921238,    0.0501744806, 1.3916802944,   -0.0055243194,    1.3619753292,    1.4406369365,    0.6529601586,    0.0538097896,    1.3821853866, 1.3870600993,    0.0040551996,    0.6600558495,    1.3550162100,    1.3081187951,   -1.7541949601,    1.3768167017, -0.6232446826,   -1.2793074919,    0.0560708725,   -0.5993356051,   -0.5857733192,   -0.6005459705,   -0.6659873442)
  
  df &lt;- data.frame(score, location, IQ)
  
  p1 &lt;- ggplot(data=df,aes(y=score,x=IQ,shape=factor(location))) + 
  geom_smooth(method = "lm", se=F) + scale_y_continuous("Test Score",limits=c(1,7)) +
  scale_x_continuous("IQ level", limits=c(-2.45, 1.45)) + 
  theme_bw() +
  theme(axis.text.x=element_text(size=rel(1.2), color = 'black'),
      axis.title.x=element_text(size=rel(1.3)),
      axis.title.y=element_text(size=rel(1.3)),
      axis.text.y=element_text(size=rel(1.2), color = 'black'),
      panel.grid.minor=element_blank(),
      panel.grid.major.x=element_blank(),
      panel.grid.major.y=element_blank(),
      axis.ticks.y = element_blank(), 
      panel.border = element_blank(),
      plot.title = element_text(size = rel(1.4))) 
  
  print(p1)
  </code></pre>
  
  <p>Whenever I try to add linetype or the manual linetype, I keep getting errors about how linetype cannot be mapped with continuous variable (only discrete).  Is there any way around this or should I be using a different function? Maybe create labels for the values in location (School 1, School 2, and School 3?) beforehand? Or should I be using some alternative to linetype? I cannot find a solution online or in Wickham's book. Thank you!</p>