[Rd] ylab in time series plot (PR#2869)

[kademan at phz.com]

You get a warning message when you specify a ylab parameter while
plotting data whose x's are POSIXct values.  Apparently the
`axis.POSIXct' method tries to reset the ylab---via the ...
parameter---after it has already been set by higher level methods.

Here is a function that illustrates the problem.

  ylabProblem <- function() {
    x <- ISOdate(2003, 4, 1:10)           # POSIXct vector
    y <- rnorm(10)
    plot(x, y, ylab = 'I am y')
  }

and this is what I get when I run it.

  > ylabProblem()
  Warning message: 
  parameter "ylab" couldn't be set in high-level plot() function 

--please do not edit the information below--

Version:
 platform = sparc-sun-solaris2.6
 arch = sparc
 os = solaris2.6
 system = sparc, solaris2.6
 status = 
 major = 1
 minor = 7.0
 year = 2003
 month = 04
 day = 16
 language = R

Search Path:
 .GlobalEnv, package:methods, package:ctest, package:mva, package:modreg, package:nls, package:ts, Autoloads, package:base