Munin could not draw graph

The documentation makes reference to two global attributes, graph_width and graph_height. Have you tried changing them? Note that these are global for a particular plugin, not for the whole munin installation; you will need to set them in the config output of a single plugin (or in munin.conf) for them to take effect.

Just to check, I put the following lines into my munin.conf:

load.graph_width 1200
load.graph_height 1000

and my load graph got absolutely massive, so I expect this will work for you, too.

You have wrong fastcgi_pass parameter, it should pass requests to spawn-fcgi just as munin's wiki says not to php-fpm.

For Ubuntu/Debian:

sudo apt-get install spawn-fcgi
spawn-fcgi -s /var/run/munin/fcgi-graph.sock -U www-data -u www-data -g www-data /usr/lib/munin/cgi/munin-cgi-graph

And dont forget to fix nginx location and restart it afterwards:

    location ^~ /munin-cgi/munin-cgi-graph/ {
        access_log off;
        fastcgi_split_path_info ^(/munin-cgi/munin-cgi-graph)(.*);
        fastcgi_param PATH_INFO $fastcgi_path_info;
        fastcgi_pass unix:/var/run/munin/fcgi-graph.sock;
        include fastcgi_params;
}