What does in show under "Source Data" for the chart?
My guess is that it may have taken the first row of data and used that as
the X values. The usual advice from Jon Peltier (the charting guru) is that
you leave the top-left hand corner cell empty, so that Excel recognises the
first row (if your data is in rows) as X values, and also recognises the
first column as series labels.
Even if you haven't got the right default behaviour when you created your
chart, you can edit the Source Data (including adding an extra series if you
need to do so).