[Rd] Bug in plot() with POSIX dates (PR#4024)

[mail at fwr.on.ca]

When I do this (highly simplified example):

plot(as.POSIXct(c("1984-01-01","1984-01-02")), c(1,2), col=2)

I get a partially red (col=2) x-axis between and including the first and last tick marks. 
Otherwise ok. Only happens with POSIXct or POSIXlt dates.

Also, POSIX dates cannot be used on the y-axis?  
POSIXlt gives an error, and POSIXct is unformatted.  

Bruce

Using X11 V4.2