Hello i've implemented a QWTPOLARPLOT arrow symbol, i've used it in my project to plot polar coordiantes. I don't know if you want to incorporate in QWTPolar but here is the code if someone needs it:

Qt Code: 
  1. void PhasorArrow::drawSymbols(QPainter *painter, const QPointF *points, int numPoints) const
  2. {
  3.  
  4.     int off = 0;
  5.     QPen pen1 = pen();
  6.     if ( pen1.width() > 1 )
  7.     {
  8.         pen1.setCapStyle( Qt::FlatCap );
  9.         off = 1;
  10.     }
  11.     painter->setPen( pen1 );
  12.  
  13.     /* Calculating the angle */
  14.     float x1 = points[0].x();
  15.     float x2 = points[1].x();
  16.     float y1 = points[0].y();
  17.     float y2 = points[1].y();
  18.     const float ang_diff = M_PI/9;
  19.     const float magnitude = 15.0;
  20.     float angulo = atan2((y2-y1),(x2-x1)) + M_PI;
  21.  
  22.  
  23.  
  24.     float right_x = x2+magnitude*cos(angulo - ang_diff);
  25.     float right_y = y2+magnitude*sin(angulo - ang_diff);
  26.  
  27.     float left_x = x2+magnitude*cos(angulo + ang_diff);
  28.     float left_y = y2+magnitude*sin(angulo + ang_diff);
  29.  
  30.     float miolo = (x2-x1)*(x2-x1)+(y2-y1)*(y2-y1);
  31.     float mag = sqrt(miolo);
  32.  
  33.     if (mag > 25)
  34.     {
  35.         QwtPainter::drawLine( painter, right_x, right_y, x2, y2 );
  36.         QwtPainter::drawLine( painter, left_x, left_y, x2, y2 );
  37.     }
  38.  
  39.  
  40.  
  41. }
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You need to extend QwtSymbol in order to do it
Cheers