title: Change axis lables in ggplot2 while glm fit is ploted
body: <p>I have the following problem:
  The data I have looks like this</p>
  
  <pre><code>NO              Income_before_taxes Income_aftere_taxes educationLevel
  1:               27757               27313              1
  2:               40147               38148              2
  3:               52240               47880              3
  4:               63061               57027              4
  5:               92409               78738              5
  6:              132985              106661              6
  </code></pre>
  
  <p>I would like to plot a glm fit of Income_aftere_taxes ~ educationLevel.</p>
  
  <p>I do this with the following code:</p>
  
  <pre><code>ggbox &lt;- ggplot(data = fullDataSet, aes(x = educationLevel, y = Income_aftere_taxes))
  ggbox &lt;- ggbox + geom_point()
  ggbox &lt;- ggbox + geom_smooth(method = "glm")
  </code></pre>
  
  <p>The result looks like this:
  <a href="https://i.stack.imgur.com/bwhE2.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/bwhE2.png" alt="enter image description here"></a> 
  However, if I want to change the axis labels of the X axis to c("lower than high school", "high school", "college", "associate degree", "bachelor", "master and PhD"), this does not work.
  To set the labels with scale_x_descrete, I need to transform the x axis input "educationLevel" to factors. This, however, destroys the glm fit.</p>
  
  <p>So, to sum up, I can either plot a glm fit or change the x axis labels. But I need both options simultaneously on one plot. Is there a way to achieve that?</p>