I have a float bar chart, with two series

The series both have the same timeline, except one series is a bunch of activities and the other is the user who performed the activities

Currently the activities show as floating bars which is what i want.  Now I want to add striplines to the back of the chart to highlight which user was operating at which times.  Only one user will be on at a time.

Is there a way to somehow loop through my series data and get these values for highlighting the background?