charting overlapping events over time

I'm trying to help a student at my university create a chart for a project. She's been observing a research subject and tracking what percent of time the subject is spending on a task. Then at a certain point, she performed an intervention, and keeps tracking the percent of time spent on the task. The data table is fairly simple--just a list of session numbers, with the percent of time for each session, except for Session 4 where the intervention happened--that one has two percentages, for percent pre-intervention and percent post-intervention. Her table needs to come out with two lines, the first starting at the first session and ending the the pre-intervention value for session 4, and the second one starting at the post-intervention value for session 4 and going to the end. It doesn't seem like this should be that complicated, but I can't get it to come out the way it should. Any advice would be greatly appreciated.

Thank you! The chart in your file is exactly what we were trying to achieve. I've passed your file along to the student, and I'm sure she'll be thrilled, but I'm still feeling baffled. For some reason, when I do the same thing, with the =NA() formula and formatting the numbers as percentages, my chart still includes the empty cells and, even more distressingly, adds the two values for Session 4 together. BTW, I'm using Excel 2010 on Windows 7.