How to draw a real point using QGraphicsItem

Question

I now have a TXT file with 100,000 two-dimensional (x, y) points.

I want to display it in a QGraphicsView, so I customized a QGraphicsItem, and the paint function draws a point.

But I found that when I zoomed in, this actually drew a rectangle:

void CustomPointItem::paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget)
{
    QPen pen{};
    pen.setColor({ 255,0,0 });
    pen.setWidth(1);
    painter->setPen(pen);
    painter->drawPoint(0, 0);
}

QRectF CustomPointItem::boundingRect() const
{
    return QRectF{ -1,1,2,2};
}

Is this how it should be? Or I did something wrong with the code?

I want to know how to better display very large number of points in Qt.