Pivot Chart Fields Change When Adding More Data

[Heather_McL]

I have the pivot chart below which breaks my data down into months/quarters. Whenever I change the data source to include more rows of data for May, I then lose the month filter and all data is plotted as individual dates. Is there any way of readding this filter?

Before changing data sourceAfter changing data source

[#2]

Perhaps there is a data quality issue? If any records of your data column contain invalid dates, this can happen.