Stacked Area Charts suddenly changed behavior

[Wild Bill]

I have a reporting tool that generates charts that combine stacked area and lines. The lines are used to show plans and forecasts. The areas show cumulative actuals to date. The charts were set up so that the areas would end in a vertical at the last month of reported actuals. Now some of my charts are showing a slope down to zero from the January value. I don't know what has changed to cause this and I don't recall how I set up the vertical end in the first place as it was a long time and an earlier version of Excel ago. Any help?