How to apply robust fitting to exclude large residuals.
  1. Select data series on left panel [Points].
  2. Select menu item <Fit><Fit Nonlinear Function>.
  3. The 'Approximation line' Wizard appears.
  4. On step 3 select Family 'Sigmoidal' and Function 'Logistic 4'.
  5. On step 4 click Find.
  6. On step 5 select line color and click Add Line.
  7. New graphs appear. See how curve fit conforms your data.
  8. Large residuals may be wrong measurements or incorrectly coded values.
  9. Let apply robust fitting to to prevent excessively perturb the function.
  10. On step 3 select Method 'Levenberg-Marquardt' and next 'Robust fitting'.
  11. On step 4 click Find.
  12. On step 5 select line color and click Add Line.
  13. Close the dialog and select menu item <View><Fitting log> to compare the fits.