Key (key)
The Key property specifies the key of the map chart item or category chart item.
Map charts display the total of the Value property for each Key.
Category charts display the total of the Value property for each Category Key divided into groups by Key.
Type: String.
The value must be unique within the chart.
This property is mandatory. An error will occur if the value is not set.
You can find the Key property in the Item category of the Properties view.