In the task pane, diagonal black triangle next to a chart category indicate All the category's options are displayed.
What is sown in the triangle
In certain software or applications that utilize a task pane for options or selections, a diagonal black triangle is commonly used as a visual indicator.
When this symbol appears next to a chart category, it signifies that all the options or categories within that specific chart category are expanded or displayed for the user to view, select, or modify.
The purpose of this symbol is to indicate that the available options within that category have been fully expanded or revealed.