轉載自博客廬州月光，向作者表示感謝
http://www.cnblogs.com/xudongliang/p/6874659.html
在R語言中，對於圖中的點來說，有很多種座標系來進行定位。
par(omi = c(1, 1, 1, 1), mai = c(1, 1, 1, 1), mfrow = c(1, 2))
plot(1:5)
box(which = "plot", col = "red", lwd = 2)
box(which = "figure", col = "red", lwd = 2)
plot(1:5)
box(which = "plot", col = "blue", lwd = 2)
box(which = "figure", col = "blue", lwd = 2)
對於圖中的某個具體的點來說，以藍色子圖中的點 （2, 2） 爲例，有很多的座標系統對這個點進行定位：
- user coordinate system : 最常用的座標系，就是x軸和y軸構成的座標系；x軸對應座標爲2， y軸對應座標爲2
- inches coordinate system : 圖像左下角爲(0, 0), 距離圖像的下邊緣和左邊緣的實際距離，單位爲 inches
- device coordinate system : 繪圖設備系統，單位爲像素
- normalize device system: 以繪圖設備的左下角爲座標原點，將長和寬歸一化成（0, 1）的區間，整個座標系統中，繪圖設備的左下角爲（0,0）， 右上角爲（1,1）
> grconvertX(2, from = "user", to = "inches")
[1] 4.634259
> grconvertY(2, from = "user", to = "inches")
[1] 2.802758
#通過gronvert 系列 函數可以看到每個點距離圖像的下邊緣和左邊緣的實際距離> par(omi = c(1, 1, 1, 1), mai = c(1, 1, 1, 1), yaxs = "i", xaxs = "i")
> plot(1:5)
> grconvertX(1, from = "user", to = "dev")
[1] 192> grconvertX(2, from = "user", to = "ndc")
[1] 0.662037
> grconvertY(2, from = "user", to = "ndc")
[1] 0.4009908相當於下面的代碼：
> grconvertX(2, from = "user", to = "inches") / par("din")[1]
[1] 0.662037
> grconvertY(2, from = "user", to = "inches") / par("din")[2]
[1] 0.4009908
#從上面的代碼看出來，normalized coordinate system 系統中點的座標實際爲在繪圖設備中的inches / 繪圖設備的長或者寬- normalized figure system: 和normalize device system 系統類似，只不過是這次相對於figure region 進行了歸一化
> grconvertX(2, from = "user", to = "nfc")
[1] 0.4537037
> grconvertY(2, from = "user", to = "nfc")
[1] 0.3613044
#相當於下面的代碼：
> (grconvertX(2, from = "user", to = "inches") - par("fin")[1] - par("omi")[2])/ par("fin")[1]
[1] 0.4537037
> (grconvertY(2, from = "user", to = "inches") - par("omi")[1])/ par("fin")[2]
[1] 0.3613044
#從上面的代碼，可以看出相對於figure region進行了歸一化- normalize plot system : 和normalize device system 系統類似，只不過是這次相對於plot region 進行了歸一化
> grconvertX(2, from = "user", to = "npc")
[1] 0.2685185
> grconvertY(2, from = "user", to = "npc")
[1] 0.2685185
相當於下面的代碼
> (2 - par("usr")[1]) / (par("usr")[2] - par("usr")[1])
[1] 0.2685185
> (2 - par("usr")[3]) / (par("usr")[4] - par("usr")[3])
[1] 0.2685185
從上面的代碼中，就可以看出來歸一化的過程