I must be missing something very obvious

 data <-ts(rnorm(120,0,1),start=1900,frequency=12)
 plot(data)
 abline(a=0,b=1)

# draws nothing
abline(a=0,b=0)  #works
abline(a=1,b=0)  #works
abline(a=0,b=.5) # doesnt work

abline will only draw if b=0
> sessionInfo()
R version 2.11.1 (2010-05-31)
x86_64-apple-darwin9.8.0

locale:
[1] en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
 [1] zyp_0.9-1         Kendall_2.1       rgdal_0.6-26      zoo_1.6-4
ncdf_1.6          R.utils_1.5.0     uncompress_1.34   R.oo_1.7.3
 [9] R.methodsS3_1.2.0 maps_2.1-4        raster_1.5-4      sp_0.9-66

loaded via a namespace (and not attached):
[1] grid_2.11.1    lattice_0.18-8 tools_2.11.1
> abline(a=0,b=0)

	[[alternative HTML version deleted]]

