title: How to find good start values for nls function?
body: <p>I don't understand why I can't have a nls function for these data.
  I have tried with a lot of different start values and I have always the same error.</p>
  
  <p>Here is what I have been doing:</p>
  
  <pre><code>expFct2 = function (x, a, b,c)
  {
    a*(1-exp(-x/b)) + c  
  }
  vec_x &lt;- c(77.87,87.76,68.6,66.29)
  vec_y &lt;- c(1,1,0.8,0.6)
  dt &lt;- data.frame(vec_x=vec_x,vec_y=vec_y)
  ggplot(data = dt,aes(x = vec_x, y = vec_y)) +  geom_point() + 
       geom_smooth(data=dt, method="nls", formula=y~expFct2(x, a, b, c),
         se=F, start=list(a=1, b=75, c=-5)
  </code></pre>
  
  <p>I have always this error:</p>
  
  <pre><code>Error in method(formula, data = data, weights = weight, ...) : 
    singular gradient
  </code></pre>