code: function() {
      if (!this.selection.prevSelection) return;
        
      var prevSelection = this.selection.prevSelection,
        lw = 1,
        plotOffset = this.plotOffset,
        x = Math.min(prevSelection.first.x, prevSelection.second.x),
        y = Math.min(prevSelection.first.y, prevSelection.second.y),
        w = Math.abs(prevSelection.second.x - prevSelection.first.x),
        h = Math.abs(prevSelection.second.y - prevSelection.first.y);
      
      this.octx.clearRect(x + plotOffset.left - lw + 0.5,
                          y + plotOffset.top - lw,
                          w + 2 * lw + 0.5,
                          h + 2 * lw + 0.5);
      
      this.selection.prevSelection = null;
    }
docs: Removes the selection box from the overlay canvas.