void MainWindow::init_plot()
{
ui->plot->xAxis->setLabel("t [hh:mm:ss]");
ui->plot->yAxis->setLabel("V [mV]");

ui->plot->addGraph(); // blue line
ui->plot->graph(0)->setPen(QPen(Qt::blue));
ui->plot->addGraph(); // red line
ui->plot->graph(1)->setPen(QPen(Qt::red));
ui->plot->addGraph(); // green line
ui->plot->graph(2)->setPen(QPen(Qt::green));

ui->plot->axisRect()->setupFullAxesBox();
ui->plot->yAxis->setRange(0, 100);

// make left and bottom axes transfer their ranges to right and top axes:
connect(ui->plot->xAxis, SIGNAL(rangeChanged(QCPRange)), ui->plot->xAxis2, SLOT(setRange(QCPRange)));
connect(ui->plot->yAxis, SIGNAL(rangeChanged(QCPRange)), ui->plot->yAxis2, SLOT(setRange(QCPRange)));

// setup a timer that repeatedly calls MainWindow::realtimeDataSlot:
connect(&dateTimer, SIGNAL(timeout()), this, SLOT(realtimeDataSlot()));
dateTimer.start(0);

}

void MainWindow::realtimeDataSlot()
{

QSharedPointer<QCPAxisTickerTime> timeTicker(new QCPAxisTickerTime);
QTime ti = QTime::currentTime();
QString tim = ti.toString("hh:mm:ss");
timeTicker->setTimeFormat(tim);
ui->plot->xAxis->setTicker(timeTicker);


static QTime time=QTime::currentTime();
ui->spinBox_2->setValue(ui->spinBox->text().toInt()+10);
ui->spinBox_3->setValue(ui->spinBox->text().toInt()-20);

// calculate two new data points:
double key = time.elapsed()/1000.0; // time elapsed since start of demo, in seconds
static double lastPointKey = 0;

if (key-lastPointKey > 0.002) // at most add point every 1s
{

  // add data to lines:
  ui->plot->graph(0)->addData(key, ui->spinBox->text().toInt());
  ui->plot->graph(1)->addData(key, ui->spinBox->text().toInt()+10);
  ui->plot->graph(2)->addData(key, ui->spinBox->text().toInt()-20);
  lastPointKey = key;
}
// make key axis range scroll with the data (at a constant range size of 8):
if(ui->pushButton->text()=="stop")
{
  ui->plot->xAxis->setRange(key, 8, Qt::AlignRight);
  ui->plot->replot();
}

}

please anyone give the solution..