Definitive Logic Advanced Gantt Chart - Groups Font Size

After trying all that are available I have started using the Advanced Gantt Chart by Definitive Logic as it best suits our requirements.

However I would like to reduce the size of the font used by the Groups field in the visual. For example, reduce the text size of the Contract Types ("Works", "Goods and Services" etc.) shown in the screenshot below.

Has anyone found a way to do this or can suggest if a work-around is possible?