ax = gca returns the current axes or chart for the current figure， which is typically the last one created or clicked with the mouse.
ax = gca 返回當前圖形的當前軸或圖表，通常是用鼠標創建或單擊的最後一個軸。
Graphics functions， such as title， target the current axes or chart. Use ax to access and modify properties of the axes or chart. If axes or charts do not exist， then gca creates Cartesian axes.
圖形功能（例如標題）以當前軸或圖表爲目標。 使用ax訪問和修改軸或圖表的屬性。 如果軸或圖表不存在，則gca創建笛卡爾座標軸。
