fixed a bug for gmtxy-image/2 when range starts by a negative value

gmtxy-image2.sh

@@ -180,7 +180,7 @@ else
 	fi
 	#根据横纵坐标的范围计算图片的长和高 我们默认图片宽度为1.5i
 	range_coor=(${range//// })
-	pic_height=`echo "scale=4; ${layouts[0]%i}  * (${range_coor[3]}-${range_coor[2]})/(${range_coor[1]}-${range_coor[0]})"|bc`
+	pic_height=`echo "scale=4; ${layouts[0]%i} * (${range_coor[3]} - ${range_coor[2]})/(${range_coor[1]} - ${range_coor[0]})"|bc`
 
 	# 输出一个bash脚本的头部到屏幕
 	if [[ ${run_command} == 0 && ${from_open} == '>' ]]; then