"xfce4-panel -h" has no reference to "--plugin-event" "xfce4-panel --plugin-event -h" does not give help on what user can do with "--plugin-event". I couldn't find any doc on Internet either.
Mass-reassign all bugs from florian@ to goodies-dev@, thanks for the maintenance work! (and sorry for the bugmail spam..)
The genmon plugin just recently received support for --plugin-event functionality to refresh the display of the plugin as if the timer had run down (see: http://git.xfce.org/panel-plugins/xfce4-genmon-plugin/commit/?id=76bd6d78220ca0c5af7beaa6700a271459d6806b). To use it, use the command: xfce4-panel --plugin-event=genmon-X:refresh:bool:true (where genmon-X is the genmon widget id (e.g. genmon-7)) I have updated the packaged README file (http://git.xfce.org/panel-plugins/xfce4-genmon-plugin/tree/README) to contain this information and will update the website (http://goodies.xfce.org/projects/panel-plugins/xfce4-genmon-plugin) shortly. As for the xfce4-panel help page not having any information regarding about this parameter, this should be filed against xfce4-panel.