R: Fitting a generalised logistic function

R: Fitting a generalised logistic function

I need some help fitting a generalised logistic function using R. For
reference, let's say the data looks roughly like this.
Initially, I tried nls using the following guide, however, I was having
trouble estimating initial parameters (i.e. receiving error messages; for
instance: singular gradient matrix at initial parameter estimates).
I next tried the package grofit (CRAN; viginette). Whilst I managed to
plot a line of best fit, it was poor as couldn't find a way to set the
lower asymptope to anything other but 0.
As I am not able to provide a working example, I don't expect a closed
solution, however, if someone could provide general advice (e.g. useful
packages, example code) that would be much appreciated.
Cheers