I want to install the software doxygen and graphvi for my research. I used the command

sudo apt-get install doxygen graphviz

then it is returning these errors:

Reading package lists... Done

Building dependency tree       

Reading state information... Done

Package graphviz is not available, but is referred to by another package.

This may mean that the package is missing, has been obsoleted, or

is only available from another source

E: Unable to locate package doxygen

E: Package 'graphviz' has no installation candidate

How can i fix this?
thanks in advance