canvas.onmousemove = function (e) {

   var loc = windowToCanvas(e);

   if (dragging) {

      restoreDrawingSurface();

      ...

      if (guidewires) {

        drawGuidewires(mousedown.x, mousedown.y);

      }

   }

};

canvas.onmouseup = function (e) {

   ...

   restoreDrawingSurface();

};