Real Time Plot Getting Slower and Slower

Hi everyone,

Im a newbie to QT and QWT and required some help. I want to draw a real time plot for sine and cosine wave but my plot get slower and slower when time passes. Any kind soul can help me out? Thank You very much.

MainWindow Code

Code:

if(plottingStatus == "play")
    {
        if(angle1 == 360)
        {
            timer->stop();
            timer->start();
        }
        else if(angle1 <= 720)
        {
            if (timer_count%3 == 0)
            {
                angle1++;
            }
            timer_count++;
 
            //calling the calculation function
            if(angle1 == 0 && rotationCount > 0)
            {
                emit toCalculate(0, timer_count, graphType, passType);
            }
            else
            {
                emit toCalculate(harmonics, timer_count, graphType, passType);
            }
 
        }
        else
        {
            plottingStatus = "stopped1";
            rotationCount ++;
            timer->stop();
            angle1 = 0;
            timer_count = 0;
        }
    }

Sine Wave Code

Code:

theta1 = ((2*n)+1) * 2 * M_PI * timer_count*0.000925925;
amplitude = (1.00/((2*n)+1)*1.00);
 
real = prev_real + (amplitude *(qCos(theta1)));
img = prev_img + (amplitude*(qSin(theta1)));
 
Y = (timer_count*0.000925925);
Y = Y;
 
yData.push_back(img);
xData.push_back(Y);
 
emit graphdata(yData, xData);

Graphing code

Code:

cSin = new QwtPlotCurve( "y = sin(x)" );
cSin->setRenderHint( QwtPlotItem::RenderAntialiased, true ); // smoothen the line of graph
 
cSin->setPen( QColor(Qt::green) );
_yData = yData;
_xData = xData;
 
cSin->setSamples(_xData, _yData);
replot();
cSin->attach( this );