Edit the plugins/Monitorix/config.php:
graph_color (black / white) ⇒ Choose between black or white
graph_width ⇒ Set the width for the graphic how they appears in adminpanel (proportional to 895 in px)
graph_height ⇒ Set the height for the graphic how they appears in adminpanel (proportional to 367 in px)
graph_enabled ⇒ Set which graph should be generated. This will modify the /etc/monitorix.conf
graph_title ⇒ Set the graphic titles
cronjob_enabled (TRUE / FALSE) ⇒ Enable or disable the cronjob to generate new graphs
cronjob_config ⇒ Set the intervall for the cronjob. More infos: man CRONTAB(5) for allowed values