Sorry if multiple posts come through, was having browser issues.
The function from ASDF you're using calls a function in CCL. After my
hunting, I ended up calling the ccl function directly and I'm not sure if
it can be adapted to the asdf one.
You'll need the file location of the dot.exe program that graphviz uses to
conver .dot to .png. It can be found in the bin folder of its instal
directory. You'll also need the full path name of the .dot file.
Now for the code:
(ccl:run-program (concatenate 'string "<dot.exe path name here> -Tpng -O
<path to .dot file>" fname)
(Please note, the <stuff> should be replaced with the appropriate info for
your system/needs. Don't type it literal.)
Using this code, you will have to specify .dot when passing a file
name(namely in the draw-city and draw-known-city functions).
I've tested it, and it works for me. Good luck!