Recently ran across this problem. Needed to fit an exponential growth model to a set of data where the curve must pass/start at the origin. Most of the exponential growth models in Fit Curve don't allow you to constrain the model parameters so that the curve will go through the origin. So I wasnt' able to do what I needed in Fit Curve. I was able to do this in Non-linear by building the model I wanted, but that is rather technical and above the level of the typical user who wants to use Fit Curve. Many chemical and physical kinetic processes y(t) start with y(0)=0. Currently only Weibull Growth & One Compartmental Oral Dose have that property.
Could either:
a) Allow for fixing and constraining parameters in Fit Curve so that you could get y(0)=0, or
b) Adding exponential growth models that go through the origin. For example, these models would be nice to have
y(t) = a(1-exp(-bt))
y(t) = a(1-exp(-bt)) + c(1-exp(-dt))