QAbstractListmodel / ListView manual positioning

dadara

In a ListView in qml, which uses the data model from a QAbstractListModel, I am trying to position the items (Rectangles) manually with data from the Model. This works in the beginning when I add the items but when I delete a ListItem the items get shifted upwards?
This is the qml:
ListView {
id: ces_cal_markers_list
objectName: "ces_cal_markers_list"
anchors.left: ces_ftuc_img.left
anchors.top: ces_ftuc_img.top
x: grid_x
y: grid_y
z: 2
interactive: true
width: ces_ftuc_img.width
height: ces_ftuc_img.height
// orientation: ListView.Vertical
model: ces_ftuc_service

    populate: Transition {
        id: dispTrans
        NumberAnimation { properties: "y"; duration: 1000 }
    }

    delegate:
        Rectangle {
            objectName: calib_id
            width: 30;
            height: 30;
            radius: width/2
            x: calib_x*ces_ftuc_img.width
            anchors.top: parent.top
            anchors.topMargin: calib_y*ces_ftuc_img.height
            color: calib_color; opacity: 0.3; border{color: "black"; width: 3 }
        }
}

deleting in subclass of QAbstractListModel in python:
def delete_calib_sample(self, row):
self.beginRemoveRows(QModelIndex(), row, row)
del self.calib_samples[row]
self.endRemoveRows()

calib_x, calib_y are the values of the list model used for positioning. Someone has an idea why the items shift after deleting an item or how I can override the automatic positioning of ListViews?
thanks in advance