changing x-axis format on chart

Good morning -

I have a chart and the x-axis is custom format i.e. Jan-11, Feb-11, Mar-11 etc...

This chart is copied to another workbook and links broken. I have deselected the 'Linked to Source' box to keep the formatting for the x-axis (before breaking the links).

However, when I apply a data table the formatting changes to text.

How can I stop this happening and keep the custom format?

Any help appreciated!

Steve