library(ggplot2)
set.seed(10)
n = 250
x = runif(n,0,5)
y_model = 3*x/(1+2*x)
y_obs = rnorm(n,y_model,0.1)
data_plot = qplot(x, y_obs) + 
            geom_line(aes(y=y_model)) + 
            theme_bw()
print(data_plot)