# File rbSnack/tkSnackUtil.rb, line 240
    def plot w1, w2, colour="#F00"
       if @worldFrame[0]<w1 and w1<@worldFrame[2] and @worldFrame[1]<w2 and w2<@worldFrame[3]
          x, y = getX(w1),  getY(w2)
          TkcOval.new(self,x-5,y-5,x+5,y+5){ fill colour} 
       end
    end