title: variable hline in ggplot with facet: discrete value supplied to continuous scale
body: <p>I am trying to include a hline similar to question (<a href="https://stackoverflow.com/questions/40350230/variable-hline-in-ggplot-with-facet">Variable hline in ggplot with facet</a>) but am having a different issue which I cannot find a working solution to.</p>
  
  <pre><code>   p1 &lt;- ggplot(excoretum, aes(x = Indmain...Dis.MutGroup, y = AgeOfOnset)) +
    geom_boxplot() + 
    geom_point() +
    geom_hline(data = excoretum, aes(yintercept = "seer_median age of diagnosis")) +
    scale_y_continuous(name = "Age of onset of disease",
                       breaks = seq(0,100, 5),
                       limits = c(0, 100)) +
   scale_x_discrete(name = "Mutation Type",
                    labels = c("Null" = "N", "Missense" = "M", "Unknown" = "U")) +
    ggtitle("Age of onset of the BAP1 extended core tumour spectrum in carriers between mutation types") +
    facet_grid(.~ICD_Grouping) +
    theme_bw()
  </code></pre>
  
  <p>That is my code, I have tried having a separate data frame, I have left_joined the data from the original separate data frame as a column with excoretum. I have made transferred the data from integer to numeric etc but regardless, whenever I try to plot this now. </p>
  
  <p>It returns:</p>
  
  <pre><code>Error: Discrete value supplied to continuous scale
  </code></pre>
  
  <p>I can put a singular hline at a value but it won't search out the hline for the facet.</p>