[R] Getting the same y-axis in a multivariate time series plot - plot(ts(...)); ylim does not do the trick

[Søren Højsgaard]

I try to obtain the same y-axis for a 2-dim time series with
 
  plot(ts(cbind(rnorm(10), rnorm(10,mean=4))),ylim=c(0,20))

but that does not work. Looking in the code for plot.ts, the ylim-argument seems to be taken care of, but not the way I expect. Can anyone help on this?
Thanks
Søren