axis函數用來在一張圖表上添加軸線，區別於傳統的x軸和y軸，axis 允許在上，下，左， 右4個方向添加軸線
以x軸為例，一條軸線包含3個元素，水平的一條橫線，叫做axis line , 刻度線， 叫做tick line, 對應的標簽 labels
基本用法：
通過side 參數設置需要添加的軸線的方向，從下邊開始，沿逆時針方向，數字為1到4
代碼示例：
par(oma = c(1, 1, 1, 1), mfrow = c(1, 4)) plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) text(x = 3, y = 3, labels = "side = 1", cex = 2) box() axis(side = 1) plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) text(x = 3, y = 3, labels = "side = 2", cex = 2) box() axis(side = 2) plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) text(x = 3, y = 3, labels = "side = 3", cex = 2) box() axis(side = 3) plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) text(x = 3, y = 3, labels = "side = 4", cex = 2) box() axis(side = 4)
效果圖如下：
參數設置：
at : 需要添加刻度的數值，默認會根據變量的取值范圍計算幾個合適的刻度，也可以手工指定
代碼示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) box() axis(side = 1, at = c(0, 2, 4, 6))
效果圖如下：
lables : 指定在刻度上需要標記的內容，默認就是刻度對應的值
代碼示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) box() axis(side = 1, at = c(0, 2, 4, 6), labels = paste(c(0, 2, 4, 6), "A", sep = ""))
效果圖如下：
tick : 邏輯值，是否顯示軸線，包括刻度線和對應的軸線， FALSE 表示不顯示
代碼示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) axis(side = 1, tick = F)
效果圖：
line : 軸線的位置
代碼示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) box() axis(side = 1, line = 1)
效果圖如下：
lwd : 設置 axis line 和 tick line 的寬度
代碼示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) box() axis(side = 1, line = 1, lwd = 2)
效果圖如下：
lwd.tick : 設置tick line的寬度
代碼示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) box() axis(side = 1, line = 1, lwd = 1, lwd.tick = 2)
效果圖如下：
lty : 設置axis line 和tick line的線條類型
代碼示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) box() axis(side = 1, line = 1, lty = 3)
效果圖如下：
col : 設置axis line 和 tick.line 的線條顏色
代碼示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) box() axis(side = 1, line = 1, col = "blue")
效果圖如下：
col.ticks : 設置tick line的顏色
代碼示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) box() axis(side = 1, line = 1, col = "blue", col.ticks = "red")
效果圖如下：
pos : 對軸線的位置進行調整，當pos 設置了對應的值之后會覆蓋line 參數的值
代碼示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F) box() axis(side = 1, pos = 1)
效果圖如下：
