How to draw a line anywhere on qwtpolar plot

Hi,
I am very new to using qt and qwt/ qwtpolar and I admit that I am really poor at mathematics :). I want to draw a line anywhere on qwtpolar plot with the two points being: point1 (radius1, angle1) and point2 (radius2, angle2). I have seen the example for spiral data plot in which the samples for radii and angles are calculated as show below:

Code:

---

class Data: public QwtSeriesData<QwtPointPolar>
{
public:
    Data( const QwtInterval &radialInterval,
          const QwtInterval &azimuthInterval, size_t size ):
        d_radialInterval( radialInterval ),
        d_azimuthInterval( azimuthInterval ),
        d_size( size )
    {
    }
 
    virtual size_t size() const
    {
        return d_size;
    }
 
protected:
    QwtInterval d_radialInterval;
    QwtInterval d_azimuthInterval;
    size_t d_size;
};
 
class SpiralData: public Data
{
public:
    SpiralData( const QwtInterval &radialInterval,
            const QwtInterval &azimuthInterval, size_t size ):
        Data( radialInterval, azimuthInterval, size )
    {
    }
 
    virtual QwtPointPolar sample( size_t i ) const
    {
        const double stepA = 4 * d_azimuthInterval.width() / d_size;
        const double a = d_azimuthInterval.minValue() + i * stepA;
 
        const double stepR = d_radialInterval.width() / d_size;
        const double r = d_radialInterval.minValue() + i * stepR;
 
        return QwtPointPolar( a, r );
    }
 
    virtual QRectF boundingRect() const
    {
        if ( d_boundingRect.width() < 0.0 )
            d_boundingRect = qwtBoundingRect( *this );
 
        return d_boundingRect;
    }
};

---

How can I implement the function "virtual QwtPointPolar sample( size_t i ) const" for finding the points ( QwtPointPolar( a, r )) of a straight line between two points on qwtpolar plot. Or if you can suggest for getting it done with QwtPlotMarker is also fine.
Please help.

vishal.