library(bbmle)
require(optimx)
x <- 0:10
y <- c(26, 17, 13, 12, 20, 5, 9, 8, 5, 4, 8)
d <- data.frame(x,y)

## breaks, don't try this
## optimx(fn=Lfn,par=c(15,6),method="Rvmmin")

m1 <- mle2(minuslogl=y~dpois(lambda=ymax/(1+x/xhalf)),
     start=list(ymax=15,xhalf=6),data=d,
     optimizer="optimx",
           method=c("BFGS","Nelder-Mead","CG"))

profile(m1)