The Group By section in the Legend Panel shows which categories that belong the feature mapped to the Group By dimension. It is also a useful way to filter your visualization by showing and hiding specific categories and bins.
This section shows every category contained in the feature. The list is ordered by the number of points in each category by default. Each category has a toggle button on the left, the name of the category in the middle, and two numbers below the category name. The two numbers show the number of visible points out of the total number of points in the dataset that are in this category.
The list is interactive and is used to quickly show or hide particular bins. Clicking on the name of the category will show exclusively that category and hide others. Clicking on the circular toggle button will toggle that bin on and off.
You can show all categories again by clicking the Reset button in the Group By section of the Legend menu.