The underlying data and lines on the chart are still correct but the labels on my chart are all showing the same value. The charts are in an automated Reporting Services report and I've never had this issue in over a year of it being run daily. This happened on the first day of December and all the labels are showing the December value.

Hello Kandis,
It looks like you want to show the Sum for the Fields!DATA.Value for the data labels. This means that you need to use “<value>” in the Data Labels Format field.

Take a look at the following Help Documentation topic: Chart > Data Model > Values Data Groupings > Data Point Formatting Commands

Best Regards,
Nevron Support Team