HIstory simplification limited to requested refs ?

Hello,

I'm trying to get gitk to draw the relationship between a bunch of integration branches.
What I'm looking for is a graph that would show which of those branches is included by
others.  --simplify-by-decoration nearly does the job, but does not omit those heads
merged into the requested branches.  Did I miss a particular git-log flag that would
allow something like --simplify-by-commandline ?

Best regards,
--
Yann