Sets or returns the formula specifying where to find the negative error value.
This formula should reference a range of cells that contains the values for the negative error. If there is only one cell referenced, then all data points will have a negative error equal to the value in that cell. If there are multiple cells referenced (e.g. A1:A5), then the first value will be used for the negative error of the first data point, the second value will be used for the second data point, and so on.
The negative error value represents the error in the negative direction. For example, if you have a data point at a Y position of 3.0, and the negative error value for the ErroyBarY is .5, then the error bar will extend from 3.0 (the data point) to 2.5 (the error value subtracted from the data point). The CustomPlusFormula specifies the error going in the other direction.
Setting this property will cause the error bar to show its negative error if it was not already doing so, and will change the ValueType property to Custom.
By default error bars will use a FixedValue ValueType, and CustomMinusFormula will return an empty string.