本函数从当前位置绘制直线段，但不包括(ｘ,ｙ)点。不包括终点坐标是为了实现多线段连接时公共点的处理，即采用起点闭区间，终点开区间的处理方法。绘制直线段函数的参数不包含颜色，直线段的颜色通过画笔来指定。(This function draws a straight line segment from the current position, but does not include (x, y) points. The end point coordinate is not used to realize the processing of common points in multi line segment connection, that is, the method of starting closed interval and ending opening interval. The parameters of the straight line segment function do not contain color, and the color of the line segment is specified by the brush.)