Make Sweave + RweaveHTML put all graphics in a specified folder

As a refinement of this question, does anyone know how to tell Sweave (or better 开发者_运维问答the driver) to put all the graphics in a specific directory when using the RweaveHTML driver from the R2HTML package? I can't find any option for that :(

Sweave responds to the prefix.string option for figures. E.g. in one recent document I use

\SweaveOpts{engine=R,eps=FALSE,echo=TRUE,prefix.string=figures/chart}

which leads to files figures/chart-chunkname.pdf where I use chunkname as the identifier in the Sweave code snippet. I suspect the same may help for R2HTML but I have not tried that driver.