API has incorrect shape. It adds an optional parameter, which is not scalable for the rest of parameters ([MS-XLSX]2.6.30 CT_DataBar has many other attributes).

The Gradient prop is onIXLConditionalFormat, where it doesn't belong. It is only for DataBar (possibly other, but only subset), not generic conditional formats.

ClosedXML uses fluent API for similar case (e.g. styles are fluentsomething.Style.Font.SetSize(14).SetBold()).

There are two interfacesIXLCFDataBarMax andIXLCFDataBarMin. Since this thing hasn't gone through API review yet, let's just addIXLCFDataBar and chain it behindIXLCFDataBarMax (currently returns void, to ensure that there are exactly two CFVO though Min and Max interface).

interface IXLCFDataBar {  bool Gradient {get;}  IXLCFDataBar SetGradient(bool value = true);}

Tests should be ClosedXml.Tests.ConditionalFormats directory. In this case, I think jut make a new ConditionalFormatDataBarTests. Test names should beExplain_what_test_does(). One test with TestHelper.CreateSaveLoadAssert should sufficie. Test file should go to ClosedXML.Tests\Resource\Other\ConditionalFormats.

Add good descriptive XML documentation for added/modified methods/props. Add user documentation for DataBar (see /doc directory). Any modification/new feature must include documentation.

Any breaking changes (in this case unification of interfaces) should be added to docs\migrations\migrate-to-0.104.rst.