If the value of the DisplayUnit property is xlCustom, the DisplayUnitCustom property returns or sets the value of the displayed units. The value must be from 0 through 10E307. Read/write Double.
Remarks
Using unit labels when charting large values makes your tick mark labels easier to read. For example, if you label your value axis in units of hundreds, thousands, or millions, you can use smaller numeric values at the tick marks on the axis.