The QtGraphs PieSeries has properties pieSize and holeSize. When the graph is rendered, it uses the hole size relative to the pie size (eg. a hole size of 1 would give a donut thickness of 0, regardless of the value of pieSize). This behavior implies that pie size and hole size should be independent.

However, it is impossible to set a hole size greater than the pie size (it will update the other value correspondingly to avoid that scenario). This implies that the hole size should be absolute, not relative to the pie size (which is not how it renders).

Reproducible example:

(Depending on the intended behavior, either the inner pie should have 0 thickness, or the outer pie should not overlap the inner pie)

import QtQuick
import QtGraphs

Window {
  width: 640
  height: 480
  visible: true title: qsTr("Graphs 2D - Pie Graph")

  GraphsView {
    id: graphsView
    anchors.fill: parent

    PieSeries {
      id: outerPie
      pieSize: 0.9
      holeSize: 0.75
      
      PieSlice {
        value: 1
        label: "label"
      }
    }

    PieSeries {
      id: innerPie
      holeSize: 1     
      pieSize: 0.74

      PieSlice {
        value: 1
        label: "label"
      }
    }

  }
}