This is working as designed
The Assignments Module columns for Task Name and Resource Name are lookups (number data type), not string fields.
The Staff Module column for Resource Name is a lookup (number data type), not a string field.
The Chart (beta) Data Categories only use String data type fields.
Therefore, if the customer wants some labels for Charts on these grids, they need to include a string field.
For Example, if they use the Task Short Name it can be added into the Grid.
Another workaround / alternative is to create a 'calculated' string field that simply concatenates the Task Name by itself in the Assignment Object and place it on the grid