I work in the telecommunications industry and received a request from a client that I cannot figure out. They purchase conferencing services by a maximum number of concurrent attendees (i.e. max 100 attendees at one time over all meetings with overage charges for greater than 100). I have a spreadsheet containing 22439 rows of data that include the start date & time, end date & time and total attendees per conference. With this data, I need to determine the number of concurrent attendees over time (YTD). Here's a sample of the data:

Start Date & Time End Date & Time TotalAttendee
1/1/11 8:19 PM 1/1/11 8:20 PM 1
1/1/11 8:27 PM 1/1/11 8:56 PM 1
1/1/11 8:28 PM 1/1/11 8:45 PM 1
1/1/11 9:37 PM 1/1/11 11:00 PM 3
1/2/11 11:59 AM 1/2/11 12:29 PM 3
1/2/11 8:30 PM 1/2/11 10:10 PM 2
1/2/11 10:05 PM 1/3/11 12:03 AM 2
1/3/11 7:30 AM 1/3/11 9:11 AM 2
1/3/11 8:36 AM 1/3/11 8:40 AM 1
1/3/11 8:41 AM 1/3/11 2:17 PM 7
1/3/11 9:03 AM 1/3/11 12:05 PM 14
1/3/11 9:11 AM 1/3/11 9:41 AM 2
1/3/11 9:29 AM 1/3/11 11:24 AM 3
1/3/11 9:58 AM 1/3/11 11:00 AM 2


In my head, I picture a stacked column graph covering a month of data, but I am uncertain on how to do this.

Any suggestions?
Thanks!
Ree