Donut charts are used to show the significance of an item in the total. Unlike pie charts, donut charts have the middle section cut out.
A donut chart requires one Size Field and one Group Field. The donut will be made from the Size Field data and Group Field determines how the slices will be split.
Please note that the Size Field needs to contain numeric fields and remember to select an Expression for this data by clicking on the field first and then choosing from the drop down.