fx,px ... указатели на объекты класса наследуемого от QGraphicsItem .
        Razmetka(ui->doubleSpinBoxA->value(),ui->doubleSpinBoxB->value(),ui->doubleSpinBoxC->value(),ui->doubleSpinBoxD->value());
        if(fx!=NULL)delete fx;
        fx=new Graphic();
        if(f1x!=NULL) delete f1x;
        f1x=new PrGraphic();
        if(Px!=NULL) delete Px;
        ...
        scene->addItem(fx);
        scene->addItem(P1x);
        scene->addItem(Px);
        scene->addItem(Rx);
        scene->addItem(f1x);

while(!vec.empty()){
        scene->removeItem(vec.at(0));
        vec.pop_front();
    }
...
text=scene->addText(t.toString(),f);
text->setPos(QPointF(i*30,600)+pog);
vec.append(text);