Different lowess curves in plot and qplot in R

Question

I am comparing two graphs with a non-parametric lo(w)ess curve superimposed in each case. The problem is that the curves look very different, despite the fact that their arguments, such as span, are identical.

y<-rnorm(100)
x<-rgamma(100,2,2)
qplot(x,y)+stat_smooth(span=2/3,se=F)+theme_bw()
plot(x,y)
lines(lowess(y~x))

There seems to be a lot more curvatute in the graph generated by qplot(). As you know detecting curvature is very important in the diagnostics of regression analysis and I fear that If I am to use ggplot2, I would reach erroneous conclusions.

Could you please tell me how I could produce the same curve in ggplot2?

Thank you

Answer 1

Or, you can use loess(..., degree=1). This produces a very similar, but not quite identical result to lowess(...)

set.seed(1)    # for reproducibility
y<-rnorm(100)
x<-rgamma(100,2,2)
plot(x,y)
points(x,loess(y~x,data.frame(x,y),degree=1)$fitted,pch=20,col="red")
lines(lowess(y~x))

With ggplot

qplot(x,y)+stat_smooth(se=F,degree=1)+
  theme_bw()+
  geom_point(data=as.data.frame(lowess(y~x)),aes(x,y),col="red")