Excel diagram limit 32000

Hi Everyone!

I have electric measurement data in each minute. That's the value of the counter, that is monotonously growing. I want to create a diagram for a month. The problem is that there are 42,300 minutes in a 30 day month, and the limit of the diagrams is 32,000 dots. In fact I don't need so many dots. What I need is the difference of the counter in every 15 minutes. Is there any formula to calculate the difference of every 15th lines? Please help! THX