At the occurence of a blank cell in the series of X values of an XY chart,
the chart automatically change to a line chart (which is useless in the
context). This drastically reduces the efficiency of Excel, for example:
1) I have to use double records of data, one containing blanks for
statistical operations and one containing #NA for XY charting
2) A single series chart can transform into charts of 20 or even hundreds of
series to circumvent the problem

This problem occurred in Excel 2003, MIcrosoft Office 2003 professional
edition