The thing i try to achieve here is calculate a new value based on existing and all result will be shown in chart, for example :
i have existing data
and then i have to calculated the next month value based on formula AVG(value), the result will be something like
at last i have to display it to the chart (picture below is not the result that i want, still missing month 4th - 12th)
Would you be able to provide a packaged workbook showing the work you've already done? If you cannot use "SuperStore" and your data is sensitive, I would check out Anonymize your Tableau Package Data for Sharing.