Wroot is correct in that the data for generating the stats are cached and saved to the database so it would be possible to have an external application query those tables, assuming you’re using an external database, to recreate the graphs you’re seeing in the monitoring plugin.
Another option you could take, aside from writing your own plugin, would be to tweak the existing monitoring plugin so that the pages you want can be accessed without requiring a login. I’ve attached a screenshot of what is possible by copying the existing Statistics page (stats-dashboard.jsp), making one change, saving it as public-stats-dashboard.jsp and adding a few lines to the plugin itself. It’s not the most attractive thing to look at, but it might be a good starting point.
If you’d like to know how to do this let me know.