It used to be. If you haven't upgraded to Excel 2007, it still is possible.
Select the point (one single click to select the series, another to select
the point). If it's a marker, put the cursor over the point and it will
become a 2-headed arrow (line chart) or 4-headed arrow (XY chart). If it's a
bar or column, put the cursor at the end of the bar and it will become a
2-headed arrow. Drag this arrow to change the value being plotted. The cell
containing the value will change. If the cell contains a formula, Excel's
Goal Seek function will be activated to let you decide which input to the
formula will result in the changed value.
- Jon