ggplot, qplot in loop

Dear List,

yes, me again trying to work with qplot ;-)

I would like to make several single plots within a loop, like this
(simplified and so on...):

trials <- c("A","B","C")
mycolours <- ("wheat","darkolivegreen","lightgreen",
       "khaki","darkseagreen","orange","chocolate4","gray75")


for (i in 1:length(trials)) {

   sql <- paste("select trial, cover_of, dateofsurvey, cover
               from mytable where trial = '",trials[i],"'
   mydata <- (sqlQuery(channel, sql))
   mytitle <- paste("Development of Cover, ", trials[i],",sep = "")
   myplot <- qplot(dateofsurvey, cover, data=mydata, geom="area",
               fill=cover_of, main = mytitle,
               ylab="projective Cover[%]", xlab = "Year")
   myplot <- myplot + scale_fill_manual(values = mycolours)
}

If I have no loop, everything works, with the loop, no graphics device is
openend, nothing happens, but the variables are filled with data (checked on
the commandline), the SQL is executed, the result from the database is
available, but nothing gets produced.

If I execute all variants by hand (all different sql-statements), the
respective graphic is produced, but put together in the loop, it does not...

OS: WinXP, R version 2.7.0 (2008-04-22)

What do I have missed?