anchor.diff:      
       super.drawChart(chart, context) ;
       
  +    if (ret == null) return [] ;
  +    
       List<List<Point>> seriesPoints = ret[0] ;
       List<String> seriesColors = ret[1] ;
       num xAxisX = ret[2] ;
  
positive.code:       for (int j = 0 ; j < data.length ; j++) {
          var d = data[j] ;
          
          var vX = d.valueY ;
          var vY = d.valueX ;
          
          double valXRatio = (vX-yValsMin) / yValsRange ;
          double valYRatio = (vY-xValMin) / xValRange ;
          
          num valX = axisXinit.x + axixXwidth * valYRatio ;
          num valY = axisYend.y + axixYheight * (1-valXRatio) ;
          
          points.add( new Point(valX,valY) ) ;
          
  
negative.code:     
      List<num> yVals = chart.getYValues() ;
      
      if (yVals.isEmpty) return null ;
      
      num yValsMin = yVals.first ;
      num yValsMax = yVals.last ;
      
  