growth.plot<-function(data,x,y,fac){
gp <- ggplot(data = data,aes(x = x, y = y))
gp <- gp + geom_point()  + facet_wrap(~ fac)
return(gp)
}

growth.plot(data=mydata, x=x.var, y=y.var,fac= fac.var)

gp1 <- ggplot(data = mydata,aes(x = x.var), y = y.var))
gp1+ geom_point()+ facet_wrap(~ fac.var) # this works

library(ggplot2)

make_plot = function(data, x, y, fac) {
    p = ggplot(data, aes_string(x=x, y=y, colour=fac)) +
        geom_point(size=3) +
        facet_wrap(as.formula(paste("~", fac)))
    return(p)
}

p = make_plot(iris, x="Sepal.Length", y="Petal.Length", fac="Species")

ggsave("iris_plot.png", plot=p, height=4, width=8, dpi=120)