nls, convergence and starting values

Bert Gunter a ?crit :

Based on a simple scatterplot of pourcma vs  transat, a 4 parameter logistic
looks like wild overfitting, and that may be the source of your problems.
Given the huge scatter, a straight line is about as much as would seem
sensible. I think this falls into the "Why ever would you want to do such a
thing?" category.

-- Bert
  

Right, well, the general idea was just to show that the "straight line" 
was the best model indeed (in the other data sets, with model 
comparison, the logistic one was clearly shown to be the best... ). Can 
the fact that convergence cannot be obtained be an acceptable and 
sufficient reason to select the null model (the straight line) ?

Patrick