void MainWindow::on_pushButton_clicked()
{
    double a,b,c;
    QCustomPlot * thisGraph = ui->customPlot;
    a = ui->lineEdit->text().toDouble();
    b = ui->lineEdit_2->text().toDouble();
    c = ui->lineEdit_3->text().toDouble();
    QVector<double> x(20);QVector<double> y(20);
    QString str, strs;
    double x0 = -b/(2*a);
    double y0 = a*pow(x0, 2.0) + a*x0 + b;
    double xMax = x0+10;
    double xMin = x0-9;
    int i = 0;
    for (double j = xMin; i<x.size(); i++)
    {
        x[i] = j;
        j++;
    }
    std::transform(std::begin(x),std::end(x),std::begin(y),
    [&](double &x){
        return a*pow(x, 2.0) + b*x + c;
    });
    ui->plainTextEdit->setPlainText(str);
    ui->plainTextEdit->setPlainText(strs);
    thisGraph->graph(0)->setData(x,y);
    thisGraph->rescaleAxes(true);
    thisGraph->replot();
}