curve fitting - How to get predicted values on Sigmoid Growth model in R -

i trying forecast future revenue bu using sigmoid growth model in r. model this:

sigmoid growth ("sigmoid" means "s-shaped"), y = a/( 1+ce^(-bx) ) + noise

my code:

x <- seq(-5,5,length=n) y <- 1/(1+exp(-x)) plot(y~x, type='l', lwd=3) title(main='sigmoid growth') 

even draw plot, don't know how future values. suppose want predict next 6 years revenue values.

make y function, , plot (plot has special support functions):

y <- function(x) 1/(1+exp(-x)) plot(y,-5,11,type="l",lwd=3)