@cv=canvas(self.default_width,self.default_height) {
      on_canvas_draw { |w,ctx|  expose(w,ctx) }
      on_canvas_button_press {|w,e|  [e.x,e.y]  }
      on_canvas_button_motion {|w,e,o| n=[e.x,e.y] ;$app.move_carto(n[0]-o[0],n[1]-o[1]) if o ;n }
      on_canvas_button_release {|w,e,o| n=[e.x,e.y] ;$app.move_carto(n[0]-o[0],n[1]-o[1]) }