XYSeries: opacity property has no effect

import QtQuick
import QtGraphs

Window {
    width: 600
    height: 400
    visible: true

    GraphsView {
        anchors.fill: parent

        LineSeries {
            width: 10
            color: "black"
            XYPoint { x:  0; y:  0 }
            XYPoint { x: 10; y: 10 }
        }
        LineSeries {
            width: 10
            color: "red"
            opacity: 0.5 // Doesn't work
            XYPoint { x:  0; y:  3 }
            XYPoint { x: 10; y:  3 }
        }
        LineSeries {
            width: 10
            color: "#77FF0000" // Works
            XYPoint { x:  0; y:  7 }
            XYPoint { x: 10; y:  7 }
        }

        axisX: ValueAxis { min: 0; max: 10 }
        axisY: ValueAxis { min: 0; max: 10 }
        theme: GraphsTheme {
            colorScheme: GraphsTheme.ColorScheme.Light
            gridVisible: false
        }
    }
}