Is there a Linux graphics program that displays man commands in a browser?

Yelp is the help viewer in GNOME.

yelp man:cgraph

On a GNU system, the program you're looking for is man.

BROWSER=firefox man --html man

Try that command (or substitute some other valid value for BROWSER=, such as, for example, cat with a redirect if you wish to save the result) and see what you get.

If you want it to be the default configuration, you'll find instructions for configuring man to your specifications in the browser window that appears.

There is xman, a graphical utility for displaying manpages.

I don't know anyone who has ever used it though. It was old an archaic already 20 years ago. For your stated use case of having manual pages displayed all the time, you'd probably be better off just opening a new terminal window and typing man something than by using xman.