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...
    ui->setupUi(this);
    scene = new QGraphicsScene(this);
    ui->graphicsView->setScene(scene);
    ...

scene->setSceneRect(0, 0, Size + 50, razmer_sceni_po_y - 30); 

QGraphicsItem *item = new Grafik(...);  

QGraphicsItem *item = new Grafik(model, Qt::red, 3, min, max);
item->setPos(QPointF(0, 200));

Grafik::Grafik(QStandardItemModel *model, const QColor &color, int stolbec, int min, int max)

QRectF Grafik::boundingRect() const
{
    return QRectF(0, 0, size_x + 5, size_y);
}

QPainterPath Grafik::shape() const
{
    QPainterPath pathik;
    pathik.addPath(path);
    return pathik;
}

void Grafik::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    Q_UNUSED(widget);

    painter->setPen(QPen(color));
    painter->drawPath(path);
}

This member is only initialized for items that have the QGraphicsItem::ItemUsesExtendedStyleOption flag set.

graphicsView->mapToScene( QRect( 0, 0,  ui->graphicsView->viewport()->width(),  ui->graphicsView->viewport()->height() ) )

item->setCacheMode(QGraphicsItem::ItemCoordinateCache); и item->setCacheMode(QGraphicsItem::DeviceCoordinateCache);

ui->graphicsView->setOptimizationFlags(QGraphicsView::DontSavePainterState);
ui->graphicsView->setViewportUpdateMode(QGraphicsView::SmartViewportUpdate);
ui->graphicsView->setCacheMode(QGraphicsView::CacheBackground);

QGraphicsItem *itema = new Grafik(model, Qt::green, 4, min, max);

void Grafik::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    Q_UNUSED(widget);

    painter->setPen(QPen(color));
    painter->drawPath(path);
}

 for (int i = 0; i <= size_x-1; i++)
    {
        x.append(my_model->item(i, stolbec)->text().toFloat());
        pointsss.append(QPointF(i,x[i]));
    }

    painter->setPen(QPen(color, 2));
    painter->save();
    painter->drawPoints(pointsss.data(), pointsss.size());
    painter->restore();