Click on the QChartView to draw crosshairs (PySide6)

Question

I want to display crosshairs on a scatterplot created by the QChart at the position where I click the mouse.

The following topic on how to display crosshairs on a graph was helpful.

How to add a crosshair to a pyqt5 graph

This works as expected at the location where no data point is displayed on the scatterplot.

However, when I click on a data point on the scatterplot, the expected crosshairs are not drawn, but small horizontal and vertical lines are drawn around the point when mouse pointer is over these points. Please see attached picture below.

Is there any way to simply display the crosshairs when the mouse is clicked on the plotArea of the chart, with or without the data point?

The following is an example for the crosshairs:

#!/usr/bin/env python
# coding: utf-8

# Reference:
# https://stackoverflow.com/questions/64446378/how-to-add-a-crosshair-to-a-pyqt5-graph
import random
import sys
from PySide6.QtCharts import (
    QChart,
    QChartView,
    QScatterSeries,
    QValueAxis,
)
from PySide6.QtCore import (
    QPoint,
    QPointF,
    QRect,
    QRectF,
    Qt,
)
from PySide6.QtGui import (
    QColor,
    QMouseEvent,
    QPainter,
    QPen,
)
from PySide6.QtWidgets import (
    QApplication,
    QMainWindow,
)


class ScatterView(QChartView):
    def __init__(self, list_data):
        super().__init__()
        self.list_data = list_data
        self.init_ui()
        self.setRenderHint(QPainter.Antialiasing)
        self._value_pos = QPoint()
        self.setMouseTracking(True)

    def generate_scatter_series(self):
        series = QScatterSeries()
        series.setMarkerShape(QScatterSeries.MarkerShape.MarkerShapeCircle)
        series.setMarkerSize(10)
        series.setPen(QPen(Qt.PenStyle.NoPen))
        for xy in self.list_data:
            series.append(*xy)
        return series

    def generate_value_axis(self):
        axis = QValueAxis()
        axis.setRange(0, 1)
        return axis

    def init_ui(self):
        chrt = QChart()
        self.setChart(chrt)
        chrt.setTitle('Scatter Plot example')
        chrt.setDropShadowEnabled(False)
        chrt.legend().hide()

        # X axis
        axis_x = self.generate_value_axis()
        chrt.addAxis(axis_x, Qt.AlignmentFlag.AlignBottom)

        # Y axis
        axis_y = self.generate_value_axis()
        chrt.addAxis(axis_y, Qt.AlignmentFlag.AlignLeft)

        # Series
        series = self.generate_scatter_series()
        series.attachAxis(axis_x)
        series.attachAxis(axis_y)
        chrt.addSeries(series)

    # _/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_
    # Override methods
    def drawForeground(self, painter: QPainter, rect: QRectF):
        super().drawForeground(painter, rect)
        if self.chart() is None or self._value_pos.isNull():
            return

        pen = QPen(QColor('red'))
        pen.setWidth(1)
        painter.setPen(pen)

        area: QRect = self.chart().plotArea()
        point: QPointF = self.chart().mapToPosition(self._value_pos)
        horiz_left = QPointF(area.left() + pen.width() / 2, point.y())
        horiz_right = QPointF(area.right() - pen.width() / 2, point.y())
        vert_top = QPointF(point.x(), area.top() + pen.width() / 2)
        vert_bottom = QPointF(point.x(), area.bottom() - pen.width() / 2)

        if area.left() <= point.x() <= area.right():
            painter.drawLine(vert_top, vert_bottom)
        if area.top() < point.y() < area.bottom():
            painter.drawLine(horiz_left, horiz_right)

    def mouseMoveEvent(self, event: QMouseEvent):
        super().mouseMoveEvent(event)
        if self.chart() is None:
            return
        point_item: QPoint = event.position().toPoint()
        point_scene: QPointF = self.mapToScene(point_item)
        if self.chart().plotArea().contains(point_scene):
            self._value_pos = self.chart().mapToValue(point_scene)
            self.setCursor(Qt.CursorShape.PointingHandCursor)
        else:
            self.unsetCursor()

        self.update()


class Example(QMainWindow):
    def __init__(self):
        super().__init__()

        # Data prep.
        list_xy_pair = [[random.random(), random.random()] for i in range(100)]

        scatter = ScatterView(list_xy_pair)
        self.setCentralWidget(scatter)
        self.resize(500, 500)
        self.setWindowTitle('ScatterView')


def main():
    app = QApplication(sys.argv)
    ex = Example()
    ex.show()
    sys.exit(app.exec())


if __name__ == '__main__':
    main()