the solutions :
The following mod_rewrite config maps the user "nagiosadmin" to NagVis:
# The following line is the really important step,
# it tells the webserver that the user "nagiosadmin" has
# successfully authenticated and is sending the request,
# regardless who is sending it really.
RewriteRule /nagvis/ - [E=REMOTE_USER:nagiosadmin]"
===> Question: in which file, you insert these lines???
A second way to "disable" the authentication is setting the variable REMOTE_USER via .htaccess file. Place a file called .htaccess with the following content in the NagVis root directory:
SetEnv REMOTE_USER nagiosusername"
===> I have inserted this line in the .htaccess file in Nagvis root directory. Nagvis still need an authentification.