Hi,
Taking a model made by HistFactory, grabbing the top level pdf (simPdf), and then calling “graphVizTree” generated a .dot file which was mostly correct. However, to use the “dot” tool - which is availible all over the place (including on lxplus @ CERN) I had to edit the file first.
Specifically:
digraph simPdf{
"simPdf" [ color=red, label="RooSimultaneous
simPdf"];
"model_twomu" [ color=red, label="RooProdPdf
model_twomu"];
"lumiConstraint" [ color=red, label="RooGaussian
lumiConstraint"];
Note the line break after “RooSimultaneous”, etc. I had to remove those line breaks in order to get this to work in the dot tool. If this could be fixed that would be great! 
Cheers, Gordon.