title: Labelling the residuals on diagnostic plots
body: <p>I have made a linear regression model in R with 3 continuous independent variables and one continuous dependent variable.  I have generated the diagnostic plots.  </p>
  
  <p>I would now like to label/colour the data points for each residual on my diagnostic plots according to the binary categorical independent variable that was not included in the model; </p>
  
  <p>i.e. when this variable = A, I want a blue dot on my diagnostic plot, 
  and when this variable = B, I want a red dot, so there will be red and blue dots on my diagnostic plots.  </p>
  
  <p>I would love some advice on how to  do this.</p>