Error with parentheses in chart labels

The series names in my chart are set to cells, some of which contain
parentheses within their text - for example: Nonconformance (Serious)

The legend incorrectly displays the name with the closing parenthesis first:
)Nonconformance (Serious

If I include the series name on the chart data, it displays correctly (which
seems to suggest a bug with the legend).

I have tried to enclose the cell values in quotes or beginning it with a
tick mark, but the error still occurs.

This occurs in both Windows Excel 2003 and Mac Excel 2004.

Any help is greatly appreciated. Thanks in advance.