tried this before but the text didn't fit in the boxes in Firefox. The solution is to use Graphviz' svg:cairo backend instead of svg. svg:cairo doesn't depend on client-side fonts.
SUU3PSXUDJCFLJDYUFSEFHKTMZGWUQZXDWP73FRAF4BS36WTRIKAC
ALIOF64IHNB3EZMOSLD5KNDZACXNN65JA6FN6YTB5R2ENSXBDMMQC
OD5FSS5AJ3XRTV5Q4UQQF6JMBT6UUL7UUOMPPUBNOM6ZMVIHW35QC
$c->response->content_type('image/png');
$c->response->content_type('image/svg+xml');
open $fh, "nix-store --query --graph @storePaths | dot -Tpng -Gbgcolor=transparent |";
open $fh, "nix-store --query --graph @storePaths | dot -Tsvg:cairo -Gdpi=100 -Gsize=20,20 -Gbgcolor=transparent |";