title: Add discrete labels to ggplot2 plot with continuous scale
body: <p>I am trying to add discrete labels to a <code>ggplot2</code> plot with a continuous scale. While there are many questions using <code>stat_function</code> (i.e., about <a href="https://stackoverflow.com/questions/10085493/ggplot2-add-a-legend-for-several-stat-functions">plotting multiple functions</a>) and many about how to use different scales, I'm having trouble understanding how to change the scale in this specific instance.</p>
  
  <p>Here is the plot:</p>
  
  <pre><code>myfun1 &lt;- function(x) (13.076-96.543)*x + (-44.056 +102.057)*x^2 + (17.856 -42.996)*x^3 + (-2.996  + 7.444)*x^4 + (0.190 -0.450)*x^5 + 100.088 + 75.215 # average vs. lowest
  myfun2 &lt;- function(x) 13.076*x -44.056*x^2 + 17.856*x^3 -2.996*x^4 + 0.190*x^5 + 100.088 # lowest
  myfun3 &lt;- function(x) (13.076-183.093)*x + (-44.056 +229.447)*x^2 + (17.856 -99.353)*x^3 + (-2.996  + 17.517)*x^4 + (0.190 -1.080)*x^5 + 100.088 + 67.115 # highest vs. lowest
  
  df &lt;- data.frame(x = c(0, 6), y = c(0, 6))
  
  myplot_weekday &lt;- ggplot(data = df, aes(x = x, y = y)) + 
    stat_function(fun = myfun3, aes(color = "Highest")) +
    stat_function(fun = myfun2, aes(color = "Lowest")) +
    stat_function(fun = myfun1, aes(color = "Average")) +
    theme_minimal() +
    scale_colour_manual("Students' Course Grade", values = c("red", "black", "blue")) +
    theme(legend.position = "top") +
    theme(text=element_text(size= 14, family= "Times")) +
    ylab("Minutes of Videos Watched") +
    xlab("Weekday")
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/GAiPt.png" rel="noreferrer"><img src="https://i.stack.imgur.com/GAiPt.png" alt="weekday plot"></a></p>
  
  <p>Instead of the continuous labels on the x-axis (0, 2, 4, and 6), I'm trying to add "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", and "Saturday," but I sense I'm approaching this the wrong way. </p>