A feature I have often made good use of in other statistics systems, is a fairly simple date marker in the graph. Simply put these markers act as a visual memory jog to show when in the graph's timeline a significant event happened, e.g. a newspaper ad, a mail-out or the boss making a speech in the canteen. It is significantly easier to spot [apparent] correlation when, for instance, a marker appears just before a peak in the graph.
In two implementations of my own [not WordPress related] I have simply made the markers vertical lines with unique colors and line types, explained in a legend. Other methods spring to mind. The important bit is, that the graph will act as a focus element in the dashboard, directly alerting you of interesting events and their [again apparent] effect in visits.
On the practical side, the markers are, in terms of data, indeed very simple. They simply have a date and a name. A very advanced version could have both a start- and an enddate if that makes sense in the individual case. The simple version alone could work magic.