Click the 'Plus' icon at the top left of the chart design area, then select the Pie Chart from the Chart Tiles, as highlighted below.
Enter a chart name, choose a unit and click "Create Chart". This will ensure that all columns in the data grid of the specified unit will be included in the Pie Chart. The chart name should describe what the visualisation is capturing.
Once the chart has been created it will then appear and will include only the columns with the specified units. These columns will appear as series on the chart. A column/ series can be removed by toggling it off in "Chart Design Mode". See the linked article below for more information.