I need some help with building a graph. My data is not generated yet, but I am trying to figure out how to get overlapping values added.

So example, I have the below

Date Range Points
11/16/2011 12:00:00 AM - 11/16/2011 12:15:00 AM 10
11/16/2011 12:00:00 AM - 11/16/2011 12:15:00 AM 20

I wanted create some kind of chart where it would add the two values instead of having it as a separate x-axis value. So the plot would be 30 throughout that time range. If there was one at 12:10-12:10, then it would only increase by 1 at that point.

I would have more data below that of course with dates/times that overlap.