I'd like to chart historical data: things like existing customers over time and existing employees over time.

Our data source has mostly transactional data: things like "this purchase made on this date" etc. I'm charting new customers by using a LOD function that figures out when their first purchase was made. What I can't do, however, is use a similar function to calculate existing customers because an existing customer might not make a purchase in the time frame I'm charting and will therefore not be displayed.

It was suggested that I solve this problem on the data source, but I'm unsure what the best way to do this would be. What would Tableau like to see? A suggestion was to create a table that contained every available date and join purchases against it, but it's unclear to me how that solves the problem.