Description:

A lot of projects expect dot -Tpng works as expected and it most likely will work if you installed libpng before installing graphviz as the post_install script detects that libpng is available for usage. Adding libpng as an optional dependency won't work as the post_install script is triggered upon installing of graphviz and not later when libpng is installed.

Expected:

Add libpng as dependency of graphviz.