While editing the material data, elongated rectangles are visible on the edges of the diagram. Those rectangles contain values - that exactly is the formatting. You can change the values by clicking on the rectangles.

Curious fact 1. You can navigate the diagram by keyboard using the TAB key. As is usually the case, the TAB key allows you to move forward and SHIFT-TAB to move back. Additionally, the ENTER key acts like the TAB key.

Curious fact 2. In the material group settings you can set auto-formatting. If so, formatting will be automatically set to this value while adding new material.

Curious fact 3. Default formatting of a material group is the same value for all edges of the material. You can, however, change this in standard size settings to make the default formatting different for each edge.