How to create dashboard graph widget that indicates when a configuration change occurred on a device?

If the config change writes something to an event log, you can parse that event log for that event.  If you can find a way to do that as a scripted datasource rather than an event source (or make an eventsource and a matching datasource that uses the Rest API to look for those Events), you could bring it in as a gauge value.  That can be set to 0 or 1.  You can then add it as a complex datapoint with another graph and just multiply it by the max value of the graph to get it to fill the graph when there's an event.  This would draw a line on the graph (technically, a little spike) when a change event occurs that is now overlaid on another datapoint graph.