[R] warnings from hist(): parameter XXX couldn't be set in high-level plot functionN

[George_Heine@blm.gov]

Hello -

This is not a real problem, just an annoyance.  Sometimes, but not always,
I get a set of strange warnings from hist().  Example follows.

############################
#Produce a histogram of start dates for a set of field measurements.

# I didn't reproduce all the dates, because not sure it's relevant, but
here's a sample.
# Note the invalid date in the fifth element.
> StartDate[sample(1:length(StartDate),20)]
 [1] "2004-08-26" "1997-09-08" "2004-08-19" "1997-09-08" "0999-07-20"
 [6] "2001-11-28" "2000-11-02" "1997-09-08" "2004-08-19" "2004-10-28"
[11] "1997-09-08" "1997-09-09" "1997-09-08" "1998-09-08" "1997-09-09"
[16] "1997-09-08" "2004-10-28" "1997-09-08" "2004-08-19" "1997-09-08"
> years<-as.Date(c(#
+    "1995-01-01","1996-01-01","1996-01-01",
+    "1997-01-01","1998-01-01","1999-01-01","2000-01-01","2001-01-01",
+    "2002-01-01","2003-01-01","2004-01-01","2005-01-01"))

> years
 [1] "1995-01-01" "1996-01-01" "1996-01-01" "1997-01-01" "1998-01-01"
 [6] "1999-01-01" "2000-01-01" "2001-01-01" "2002-01-01" "2003-01-01"
[11] "2004-01-01" "2005-01-01"

>  TimeSpan<-as.integer(difftime(StopDate,StartDate))
Warning message:
NAs introduced by coercion
#  Note:  NAs correspond to 3 invalid dates, apparently typos

> h<-hist(StartDate[TimeSpan>0 & StartDate>years[1]], breaks=years, freq=T,
+       main="",xlab="",ylab="number of sites", col="grey")
Warning messages:
1: parameter "main" couldn't be set in high-level plot() function
2: parameter "ylab" couldn't be set in high-level plot() function
3: parameter "main" couldn't be set in high-level plot() function
4: parameter "ylab" couldn't be set in high-level plot() function
#########################################3

The resulting histogram was generated, and the plot labels main, xlab,
ylab, were as expected.  Just curious about what the warning messages mean.
Why are there two warnings each about 'main' and 'ylab', and none about
'xlab'?   Another puzzling thing is that this message does not always
occur,
Searched the FAQ and the mail archives and couldn't find anything helpful.

 By the way, is there a way to use seq() to generate something like the
"years" sequence above?

I'm using R 2.0.1 (precompiled version for windows.)

Thanks for your help !
<>=<>=<>=<>=<>=<>=<>=<>=<>=<>=<>
George Heine, PhD
Mathematical Analyst
National IRM Center
U.S. Bureau of Land Management
voice   (303) 236-0099
fax       (303) 236-1974
cell      (303) 905-5382
pager   gheine at my2way.com
<>=<>=<>=<>=<>=<>=<>=<>=<>=<>=<>