[R] Adding a method to a generic in another package

[Jeffrey J. Hallman]

I am the maintainer of the 'tis' package.  One of the functions in my package
is 'nberShade'.  A user wants to make nberShade generic, with the old version
renamed as nberShade.default, all of which is fine with me.  And he wants to
add a new method, nberShade.ggplot, which works for objects of class ggplot.
He also wants to add a method fortify.tis for the generic fortify defined in
ggplot2. 

The nberShade.ggplot method uses a bunch of other functions from the ggplot2
package, and it's first line is

require("ggplot2")

>From what he tells me, this function works.  

Where I'm having trouble is figuring out what I have to do to get the tis
package to pass R CMD check.  I really don't want to force users of the tis
package to have to install ggplot2.  What can I do?  Is it enough to have 

Imports: ggplot2

in the DESCRIPTION file and 

import(ggplot2)

in the NAMESPACE file?  I've done that, but I still get this warning from R
CMD check:

* checking for unstated dependencies in R code ... WARNING
'library' or 'require' calls not declared from:
  ggplot2
See the information on DESCRIPTION files in the chapter 'Creating R
packages' of the 'Writing R Extensions' manual.

Well, I did read the manual, and it seems to say that what I'm doing is OK. So
why am I getting the warning?

-- 
Jeff