Virtual pens would allow a user to create ad hoc expressions/calculations based on existing tags that are in the trend only. This would allow users another avenue to explore / manipulate their information for viewing. Essentially, this is requesting to expand the ability of calculated pens for a more customizable experience.

Ideally, these "virtual pens" could leverage the same functionality seen in expression tags. Maybe the UI could even have a "Build Tag" button that could be used to take the final product in the chart and build it as an expression tag if so desired by the end user.