changing the Y axis of Plot makes the program unreponsive

I am trying to plot a random value in QWTPlot.
when I set the max value for the Y axis with the Max value of the randomly generated value, the program becomes unresponsive.(i.e)ui->SamplePlot->setAxisScale(QwtPlot::yLeft,0,MaxValue,MaxValue/2);//not working
If I set the max value of Y axis to three times of Max value of randomly generated value it is working fine. (i.e)ui->SamplePlot->setAxisScale(QwtPlot::yLeft,0,MaxValue*3,(MaxValu e*3)/2);
Below is the program I used for that
//mainwindow.cpp

Code:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <windows.h>
 
#include <qwt_plot_grid.h>
#include <qwt_plot_layout.h>
 
#include <qwt_plot_curve.h>
 
#include <qwt_painter.h>
 
 
QwtPlotGrid *Grid;
QwtPlotCurve *curveA;
bool updatePlotFlag = false;
HANDLE hDisplayThread;
int  MaxValue;
double c[1024];
double XValue[1024] = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,301,302,303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,335,336,337,338,339,340,341,342,343,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,366,367,368,369,370,371,372,373,374,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390,391,392,393,394,395,396,397,398,399,400,401,402,403,404,405,406,407,408,409,410,411,412,413,414,415,416,417,418,419,420,421,422,423,424,425,426,427,428,429,430,431,432,433,434,435,436,437,438,439,440,441,442,443,444,445,446,447,448,449,450,451,452,453,454,455,456,457,458,459,460,461,462,463,464,465,466,467,468,469,470,471,472,473,474,475,476,477,478,479,480,481,482,483,484,485,486,487,488,489,490,491,492,493,494,495,496,497,498,499,500,501,502,503,504,505,506,507,508,509,510,511,512,513,514,515,516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639,640,641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,666,667,668,669,670,671,672,673,674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719,720,721,722,723,724,725,726,727,728,729,730,731,732,733,734,735,736,737,738,739,740,741,742,743,744,745,746,747,748,749,750,751,752,753,754,755,756,757,758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797,798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822,823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838,839,840,841,842,843,844,845,846,847,848,849,850,851,852,853,854,855,856,857,858,859,860,861,862,863,864,865,866,867,868,869,870,871,872,873,874,875,876,877,878,879,880,881,882,883,884,885,886,887,888,889,890,891,892,893,894,895,896,897,898,899,900,901,902,903,904,905,906,907,908,909,910,911,912,913,914,915,916,917,918,919,920,921,922,923,924,925,926,927,928,929,930,931,932,933,934,935,936,937,938,939,940,941,942,943,944,945,946,947,948,949,950,951,952,953,954,955,956,957,958,959,960,961,962,963,964,965,966,967,968,969,970,971,972,973,974,975,976,977,978,979,980,981,982,983,984,985,986,987,988,989,990,991,992,993,994,995,996,997,998,999,1000,1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1011,1012,1013,1014,1015,1016,1017,1018,1019,1020,1021,1022,1023,1024};
 
int DisplayProcess();
 
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    updatePlotArea();
}
 
MainWindow::~MainWindow()
{
    delete ui;
}
 
 
void MainWindow::updatePlotArea()
{
    Grid = (new QwtPlotGrid());
    Grid->enableYMin(true);
    Grid->attach(ui->SamplePlot);
    /*ui->SamplePlot->plotLayout()->setAlignCanvasToScales(true);*/
    ui->SamplePlot->setAxisScale(QwtPlot::yLeft,0,10000,10000/2);
    ui->SamplePlot->setAxisScale(QwtPlot::xBottom,0,1024,1024/2);
    ui->SamplePlot->setAxisMaxMinor(QwtPlot::xBottom, 0);
    ui->SamplePlot->setAxisMaxMinor(QwtPlot::yLeft, 4);
    Grid->setPen(QPen(Qt::white, 0.0, Qt::DotLine));
 
    /*create curve for channel A */
    curveA = new QwtPlotCurve();
    curveA->setStyle(QwtPlotCurve::Lines);
    curveA->setPen(QPen(Qt::yellow));
    curveA->setRenderHint(QwtPlotItem::RenderAntialiased, true);
    curveA->setPaintAttribute(QwtPlotCurve::ClipPolygons, false);
    curveA->attach(ui->SamplePlot);
 
}
 
bool MainWindow::event(QEvent *e)
{
    if (e->type() == QEvent::User + 2)
    {
        ui->SamplePlot->setAxisScale(QwtPlot::yLeft,0,MaxValue*3,(MaxValue*3)/2);
        //ui->SamplePlot->setAxisScale(QwtPlot::yLeft,0,MaxValue,MaxValue/2);//not working
 
        curveA->setSamples(XValue,c,1024);
        ui->SamplePlot->replot();
 
        return true;
    }
    return QMainWindow::event(e);
}
 
void MainWindow::on_Start_clicked()
{
    if (updatePlotFlag == false)
    {
        updatePlotFlag = true;
        hDisplayThread = CreateThread(NULL,0,(LPTHREAD_START_ROUTINE)&DisplayProcess,0,0,NULL);
        ui->Start->setText("Stop");
    }
    else
    {
        updatePlotFlag = false;
        CloseHandle(hDisplayThread);
        ui->Start->setText("Start");
    }
 
}
 
 
void getInput()
{
    double y;
    int i;
    double Max = 0;
    for(i = 0; i<1024; i++)
    {
        y = 10*(sin(0.01*i*(100))+sin(0.075*i)+cos(0.5*i)+0.06*(rand()-0.5*(100)));
        c[i] = y;
        if(y >Max)
            Max = y;
    }
    MaxValue = Max;
}
 
 
int DisplayProcess()
{
    while(updatePlotFlag == true)
    {
        getInput();
        Sleep(50);
        DataAvailableEvent *e = new DataAvailableEvent();
        QApplication::postEvent(mainWindow, e);
    }
    return 1;
}

//end mainwindow.cpp

//main.cpp

Code:

#include <QtGui/QApplication>
#include "mainwindow.h"
MainWindow *mainWindow;
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    mainWindow = new MainWindow() ;
    mainWindow->show();
 
    return a.exec();
}

//end main.cpp

mainwindow.h

Code:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
#include <QApplication>
 
namespace Ui {
    class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
protected:
    bool event(QEvent *e);
 
private slots:
    void on_Start_clicked();
 
private:
    Ui::MainWindow *ui;
    void updatePlotArea();
};
 
class DataAvailableEvent :public QEvent
{
public:
    DataAvailableEvent() : QEvent((QEvent::Type)(QEvent::User + 2)){}
 
};
 
extern MainWindow *mainWindow;
 
 
#endif // MAINWINDOW_H

//end mainwindow.h