Charts show wrong result when second workbook is opened

Situation:
A single workbook containing charts is opened:
results look well

Now when I open a second workbook (alt+tab to folder, double click workbook) , the charts (in both workbooks) suddenly show the wrong results:
(See attached thumbnails)

It does not make a change when I select the view side by side option.

Please, can someone explain why does this happen and how I can fix this?

Thank you,
Mark

Plot contains trendline which fits the data and is displayed together with equation on the plot

Hope this helps..

Additional comment and workaround:
If I move the excel file that contains the data used for the plots to a subfolder, the graphs won't disposition when I open one or more additional workbooks.

I can live with that, but still I do not understand why..