How can I make doxygen create full inheritance diagrams across multiple projects?

Question

When using doxygen to generate C++ inheritance diagrams, I've noticed that the diagrams can be incomplete.

If several projects are documented using tag files to allow cross-referencing, doxygen will successfully show all base classes that exist in other tag files, but it will not show derived classes if they are in other tag files. The derived classes shown are always only the classes that exist in the current project.

Example project 1 code:

class A {};

class B : public A {};

class C : public A {};

Example project 2 code:

#include <proj1.h>

class D : public A {};

class E : public A {};

class F : public E {};

An inheritance diagram of A from project 1, using a tag file from project 2 - where is D, E & F?

An inheritance diagram of E from project 2, using a tag file from project 1 - parent class A is shown.

Is there a setting to tell doxygen to generate complete inheritance graphs across projects when tag files are used?