pdfnup and page size

I use a combination of pdftk and pdfnup, from the pdfjam package to arrange (PDF) graphs produced by ctioga into an array for printing. However, I find that a simple

pdfnup --nup 3x3 concatenated_graphs.pdf

does not produce an A4 output (even with --paper a4paper), but a PDF who has the same size as individual graphs. Painful. It did take me long to realize it is a feature of pdfnup, who asks pdfTeX to have the paper size adjusted to that of the input when the layout is symmetric and no orientation has been provided. To disable that, and get a real A4 paper, specify the orientation:

pdfnup --orient portrait --nup 3x3 concatenated_graphs.pdf