Allow users to add a fixed, free form data element to a chart containing a similar dimension

As a business user, I would like the ability to specify a fixed data element within a chart already containing the same data element. 

Examples include:

* A specific date in a year that is important to the company (eg renewal date, project completion)
* A calculated field to compare the industry average of metrics vs the displayed metric(s).

If this is an additional date field, I would expect to see a highlighted period of time
If this is an additional metric, I would expect to see an additional dotted line to distinguish its special characteristics.

Complexity could be limited by only offering this functionality for specific kinds of visualizations, such as line graphs (or graphs with dates, as per the date example).