Quite a few prior posts on forecasting, have you reviewed those yet?
The above is likely the closest fit, believe there is an example app in the attached file too. If you wish further help, you will likely need to attach what you have in the way of current QVW, and if you have sensitive data, you can scramble it prior to posting by using Desktop Client and Settings\Document properties\Scrambling tab to scramble any sensitive fields in the app. Generally in these types of posts you need to provide at least some sample data and expected output desired in order for folks to offer up much help.
The other area of Community you can use for further ideas on how to go about things is the Design Blog area, but there are no forecasting posts out there, but lots of other how-to related items you may need for this one.
The only other place to have a look would be https://market.qlik.com/
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