Multiple use of QGraphicScene

[nish]

if you dont want to make it a member than you can access the scene by calling

//draw dot
void Unknown::drawDot()
{
    //scene->addEllipse(50, 50, 1, 1, QColor(255, 0, 0), Qt::NoBrush);
    ui->graphicsView->scene()->addEllipse(50, 50, 1, 1, QColor(255, 0, 0), Qt::NoBrush);
    //We already set this in previous function so no need now
   //ui->graphicsView->setScene(scene);
}

//draw dot
void Unknown::drawDot()
{
    //scene->addEllipse(50, 50, 1, 1, QColor(255, 0, 0), Qt::NoBrush);
    ui->graphicsView->scene()->addEllipse(50, 50, 1, 1, QColor(255, 0, 0), Qt::NoBrush);
    //We already set this in previous function so no need now
   //ui->graphicsView->setScene(scene);
}

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but as suggested you should learn to make a class.