Be more flexible on the graphviz dependency : replaced the "Depends" by "Recommends" in debian control file. (see https://www.debian.org/doc/debian-policy/ch-relationships.html, section 7.2 Binary Dependencies) :

The Depends field should be used if the depended-on package is required for the depending package to provide a significant amount of functionality. 


Recommends

    This declares a strong, but not absolute, dependency.

    The Recommends field should list packages that would be found together with this one in all but unusual installations.