Why does converting line chart to area chart cause label errors?

[Ironman]

Hi

I have a chart that displays 2 sets of data perfectly in the Line Chart format.

However, when I change one of them to the Area Chart format, the points in the Area Chart display either nonsensical data or point numbers.

Is there a simple reason for this?

Apologies for not posting the chart but I don't know how.

Many thanks!