Bring up the Format Series dialog, by double clicking on the series, by right clicking on the series and choosing Selected Object, by selecting the series and choosing Selected Data Series from the Format menu, or by selecting the series and clicking the shortcut, Ctrl 1 (numeral one).
The data is set up so that, for example, cells C2 and D2 have the values for the positive and negative horizontal (X) error bars for the point defined by X and Y values in A2 and B2.
Cells E2 and F2 have the values for the positive and negative vertical (Y) error bars for this point.
If you select a single value for your custom error bars, this single value will be applied to all points in the series.
A whole set of custom error bar values can be added to an entire series in one operation.
The chart itself is easy: create an XY chart using the data in columns A and B.
The protocols for adding custom error bars differ between Classic Excel and new Excel.
The edit box is so narrow, that you cannot see the entire expression at once, and it will be difficult to find this error. Don’t completely clear an entry box, because Excel will think you simply forgot and it will retain the previous value.
It’s easy to add error bars in Excel 97 through 2003.
Move the dialog so it does not cover the range containing your custom values, then click on the Custom option button, and click on Specify Value.
A small child dialog appears with entry boxes for selection of the custom error bar values.
To discover these features in Excel 2007, select the chart and navigate to the Chart Tools Layout contextual tab.