Wave diagram in QT (functionality issues in logic)

Wave diagram in QT.. There was some functionality issues in logic. I tried a wave diagram logically using QPainter. Wave diagram is drawn perfectly but there was one line has drawn.. Any one help me to solve this problem.

//dialog.cpp

#include "dialog.h"
#include "ui_dialog.h"
#include <QtMath>
#include <QDebug>
#include <QDialog>
#include <QTGui>
#include <QtCore>


Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);
}

Dialog::~Dialog()
{
    delete ui;
}

void Dialog::paintEvent(QPaintEvent *e)
{
    e->accept();

    float scale = 40;
    //boolean negativeX = true;
    float width = 500;
    float height = 108;

    QPainter painter(this);

    QPen linepen(Qt::red);
    linepen.setWidth(2);
    QPoint p1;
    QPoint p2;

    painter.setPen(linepen);

    float xx, yy, dx=4, x0=width / 2, y0=height / 2;

    //float iMax = (width - x0) / dx;
    float iMax = 63;

    //float iMin = negativeX ? -x0 / dx : 0;
    float iMin = -63;

    for (int i=iMin;i<=iMax;i++) {
           float x = x0+xx;
           float y = y0-yy;
         p1.setX(x);
         p1.setY(y);

         xx = dx*i;
         float xscl = xx/scale;
         yy = scale * qCos( 3* xscl );

           x = x0+xx;
           y = y0-yy;

         p2.setX(x);
         p2.setY(y);

         painter.drawLine(p1, p2);

     }

   }

// dialog.h

protected:
    void paintEvent(QPaintEvent *e);

// main.cpp

QApplication a(argc, argv);
Dialog w;
w.show();