Add Option to Multipane Graph

I operate an opensource system called EmonCMS to record energy usage for a client ([login to view URL]) EmonCMS uses the following work flow: 1. data from a remote energy meter posts data to EmonCMS using JSON in the URL String (this is called and input in EmonCMS) 2. EmonCMS then allow for the editing of the incoming values and is then exposed to a dashboard (this is called a feed in EmonCMS, each feed has its own table in mysql) 3. a feed is then used in displaying data on graphs and gauges. I need to add the following functionality to the system: See screenprint attached - I would like to add an option to the existing multigraph to show a peak value for the selected time series. The purpose for this is for a customer to see what the highest point that a monitored value reached during the time series. Thanks Chris