R help - May 2009 - Problem with adding labels in ggplot2

I apologize for not pasting a complete example, but the data-set is too large,
so I hope someone can help me just by description of symptoms. 

I define a generic plot object name (note the missing y=.. in aes()) to
plot different y-values against the same set of x-values.

p.b4.generic.wg <-
  ggplot(subset(b4.all.medians, ncpus==8, TRUE), aes(x=wg)) +
  geom_line(aes(linetype=graph, group=interaction(graph,nwrk,ncpus))) +
  geom_point(aes(shape=nwrk)) +
  scale_shape(name="# of workers") +
scale_linetype(name="Workload") +
  xlab("Work division") + ylab("N/A")

Now,

p.b4.stealspins <- p.b4.generic.wg + aes(y=v.stealspins / v.realtime) +
  ylab("Steal rate")

draws the correct graph EXCEPT that the y-axis label is wrong.  The ylab() is
ignored and the y-label is set to "v.stealspins / v.realtime".  What
am I
doing wrong here?

On Wed, May 27, 2009 at 01:40:52PM +0200, Zeljko Vrba
wrote:
>
> I apologize for not pasting a complete example, but the data-set is too
large,
> so I hope someone can help me just by description of symptoms. 
> 
-snip-

I have solved the problem by introducing an artificial variable in the original
plot specification and plotting the data using the %+% operator, e.g.:

p.b4.idletime <- p.b4.generic.wg %+%
  within(b4.all.medians.8, { y <- v.idletime }) +
    ylab("Idle time (s)")