Dates are whole numbers.What you see is just the format.
So to example 40250 is 13/03/2010 and today's date is 42305.
If your Date1 is less that Date2 you will get values less that 0 -divide that by 365 you will get fraction.
Post your data or qvf file to see what is going on
Try like this, Qlikview internally stores dates as numbers.
(Num(Date1) - Num(Date2)) / 365
If dates are not in date format then you have convert it by using Date#() like below
(Date#(Date1, 'MM/DD/YYYY') - Date#(Date2, 'MM/DD/YYYY')) / 365
Replace MM/DD/YYYY with your actual date format.
Hope this helps you.
Thank you all for your quick responses. I have tried each of your suggestions but no luck, I still just get a flat line. The date fields I am using are currently in Date format via Excel. So Im not sure why its not working. Unfortunately the data I am using is confidential so I wouldn't be able to post the full application. But I have pulled just what is used for this chart for a sample qvf file, uploaded to google drive - Date Calc Test.qvf - Google Drive
Thanks for your assistance,