An Heroku buildpack to install graphviz.
Execute following command to use buildpack-graphviz as buildpack:
heroku config:add BUILDPACK_URL=https://github.com/jeluard/heroku-buildpack-graphviz.git
You certainly want to do something more than just using graphviz. You will then have to use other build packs. This can be achieved with buildpack-graphviz.
Add all needed buildpacks to your .buildpacks:
https://github.com/jeluard/heroku-buildpack-graphviz.git
... # all others buildpacks
Use buildpack-multi as buildpack:
heroku config:add BUILDPACK_URL=https://github.com/ddollar/heroku-buildpack-multi.git
Due to a buildpack-multi limitation variables exported by all but last defined buildpack will be ignored. An easy workaround is to define graphviz buildpack first then manually export variable:
heroku config:add LD_LIBRARY_PATH=.graphviz/lib/