title: Calculation of R^2 value for a non-linear regression
body: <p>I would first like to say, that I understand that calculating an R^2 value for a non-linear regression isn't exactly correct or a valid thing to do. </p>
  
  <p>However, I'm in a transition period of performing most of our work in SigmaPlot over to R and for our non-linear (concentration-response) models, colleagues are used to seeing an R^2 value associated with the model to estimate goodness-of-fit. </p>
  
  <p>SigmaPlot calculates the R^2 using 1-(residual SS/total SS), but in R I can't seem to extract the total SS (residual SS are reported in summary). </p>
  
  <p>Any help in getting this to work would be greatly appreciated as I try and move us into using a better estimator of goodness-of-fit.</p>
  
  <p>Cheers.</p>