1. Add method CategoricalErrorBarsBuilder.Field(string) that will indicate the name of the field containing the value for the error bar.  This value would be a relative difference (not absolute high/low).  There is a "Fields(string,string)" method but "Field(string)" for difference value is missing.

2. Provide way to render half (top or bottom) of error bar.  For example, if only CategoricalErrorBarsBuilder.HighField(string) is assigned, then render top half of error bar.  Or maybe some other setting.  Often it is desirable to show only the side of the error bar that is above (or below, if the bar represents a negative value) the bar marker so that the other half is not rendered within the bar marker.