[R] why won't rq draw lines?

[david_foreman@doctors.org.uk]

I've been trying to draw quantile linear regression lines across a scatterplot of my data using 

attach(forrq)
     plot(PREGNANT,DAY8,xlab="pregnant EPDS",ylab="postnatal EPDS",cex=.5)
     taus <- c(.05,.1,.25,.75,.9,.95)
     xx <- seq(min(PREGNANT),max(PREGNANT),100)
     for(tau in taus){
             f <- coef(rq(DAY8~PREGNANT,tau=tau))
             yy <- (f[1]+f[2]*xx)
             lines(xx,yy)
             }
which is simply the method from the help file with my dataset attached, and the variable names substituted where appropriate.  I get the scatterplot, but no lines.  Any ideas about what's going on? or wrong?


_______________________________________________________________________
Most doctors use http://www.Doctors.net.uk e-mail.
Move to a free professional address with spam and virus protection.