diagram → expression Relation

File in src/core/diagram	Includes file in src/core/expression
qgsdiagram.h	qgsexpression.h
qgshistogramdiagram.cpp	qgsexpression.h
qgspiediagram.cpp	qgsexpression.h
qgsstackedbardiagram.cpp	qgsexpression.h
qgstextdiagram.cpp	qgsexpression.h