By default, the FormatHistogramValues property is empty. The bar's histogram value gets formatted, if the FormatHistogramValues expression is not empty and valid. You can use the FormatHistogramValues property to customize the bar's values in the histogram. The bar is represented in the control's histogram, if its HistogramPattern / HistogramColor property is defined. Use the HistogramType property to specify the type of the graph to be displayed in the histogram for specified bar. The ResizeUnitScale property determines the refinement for the histogram part.

The following screen show shows how the bar's value/effort is displayed in the control's histogram by default:

The following screen show shows how the formatted bar's value/effort is displayed in the control's histogram ( FormatHistogramValues property is "(value format `1`) + ` units`"):

 

The value keyword in the FormatHistogramValues property indicates the value to be formatted (ItemBar(exBarEffort)). 

This property/method supports predefined constants and operators/functions as described here.