Hi,
A each series in the chart has a formula, appears in the formula bar
when the series is selected, for example,
=SERIES(,Sheet1!$B$3:$B$6,Sheet1!$C$3:$C$6,1)
This formula is restricted to a length of 1024 characters.
Formula normally gets to long if the cell are non-contiguous, for
example using same data range but selected whilst holding CTRL ,
=SERIES(,(Sheet1!$B$3,Sheet1!$B$4,Sheet1!$B$5,Sheet1!$B$6),(Sheet1!$C$3,Sheet1!$C$4,Sheet1!$C$5,Sheet1!$C$6),1)
If this is the problem try placing the data in a contiguous block.
Cheers
Andy