This page explains how to interpret the graphs that are generated by doxygen.
Consider the following example: 
    1 /*! Invisible class because of truncation */
 
    4 /*! Truncated class, inheritance relation is hidden */
 
    5 class Truncated : public Invisible { };
 
    7 /* Class not documented with doxygen comments */
 
    8 class Undocumented { };
 
   10 /*! Class that is inherited using public inheritance */
 
   11 class PublicBase : public Truncated { };
 
   13 /*! A template class */
 
   14 template<class T> class Templ { };
 
   16 /*! Class that is inherited using protected inheritance */
 
   17 class ProtectedBase { };
 
   19 /*! Class that is inherited using private inheritance */
 
   20 class PrivateBase { };
 
   22 /*! Class that is used by the Inherited class */
 
   25 /*! Super class that inherits a number of other classes */
 
   26 class Inherited : public PublicBase,
 
   27                   protected ProtectedBase,
 
 This will result in the following graph:
The boxes in the above graph have the following meaning: 
- 
A filled gray box represents the struct or class for which the graph is generated. 
- 
A box with a black border denotes a documented struct or class. 
- 
A box with a grey border denotes an undocumented struct or class. 
- 
A box with a red border denotes a documented struct or class forwhich not all inheritance/containment relations are shown. A graph is truncated if it does not fit within the specified boundaries. 
The arrows have the following meaning: 
- 
A dark blue arrow is used to visualize a public inheritance relation between two classes. 
- 
A dark green arrow is used for protected inheritance. 
- 
A dark red arrow is used for private inheritance. 
- 
A purple dashed arrow is used if a class is contained or used by another class. The arrow is labeled with the variable(s) through which the pointed class or struct is accessible. 
- 
A yellow dashed arrow denotes a relation between a template instance and the template class it was instantiated from. The arrow is labeled with the template parameters of the instance.