你如何在 R 中的多图形环境中画一条线? [英] How do you draw a line across a multiple-figure environment in R?

问题描述

举个很简单的例子，mfrow=c(1,3);每个数字都是不同的直方图；我将如何绘制一条横穿所有 3 个数字的水平线(类似于abline(h=10))?(也就是说，即使是它们之间的边距.)显然，我可以为每个图形添加一个 abline，但这不是我想要的.我可以想到一种非常复杂的方法来做到这一点，真的只有一个图形，并使用 polygon 等在其中绘制每个图形".那太荒谬了.难道没有简单的方法可以做到这一点吗?

Take a very simple example, mfrow=c(1,3); each figure is a different histogram; how would I draw a horizontal line (akin to abline(h=10)) that went across all 3 figures? (That is, even the margins between them.) Obviously, I could add an abline to each figure, but that's not what I want. I can think of a very complicated way to do this by really only having 1 figure, and drawing each 'figure' within it using polygon etc. That would be ridiculous. Isn't there an easy way to do this?

推荐答案

正如@joran 所指出的，网格 图形系统可以更灵活地控制单个设备上多个绘图的排列.

As @joran noted, the grid graphical system offers more flexible control over arrangement of multiple plots on a single device.

这里，我首先使用grconvertY()来查询y轴上高度为50的位置以标准化设备坐标"为单位.(即作为绘图设备总高度的比例，0=底部，1=顶部).然后我使用 grid 函数来: (1) 推送一个填充设备的 viewport；(2) 在 grconvertY() 返回的高度处绘制一条线.

Here, I first use grconvertY() to query the location of a height of 50 on the y-axis in units of "normalized device coordinates". (i.e. as a proportion of the total height of the plotting device, with 0=bottom, and 1=top). I then use grid functions to: (1) push a viewport that fills the device; and (2) plot a line at the height returned by grconvertY().

## Create three example plots
par(mfrow=c(1,3))
barplot(VADeaths, border = "dark blue") 
barplot(VADeaths, border = "yellow") 
barplot(VADeaths, border = "green") 

## From third plot, get the "normalized device coordinates" of 
## a point at a height of 50 on the y-axis.
(Y <- grconvertY(50, "user", "ndc"))
# [1] 0.314248

## Add the horizontal line using grid
library(grid)
pushViewport(viewport())
grid.lines(x = c(0,1), y = Y, gp = gpar(col = "red"))
popViewport()

编辑:@joran 询问如何绘制一条从第一个图的 y 轴延伸到第三个图中最后一个条形边缘的线.这里有几个替代方案:

EDIT: @joran asked how to plot a line that extends from the y-axis of the 1st plot to the edge of the last bar in the 3rd plot. Here are a couple of alternatives:

library(grid)
library(gridBase)
par(mfrow=c(1,3))

# barplot #1
barplot(VADeaths, border = "dark blue") 
X1 <- grconvertX(0, "user", "ndc")
# barplot #2
barplot(VADeaths, border = "yellow") 
# barplot #3
m <- barplot(VADeaths, border = "green") 
X2 <- grconvertX(tail(m, 1) + 0.5, "user", "ndc") # default width of bars = 1
Y <- grconvertY(50, "user", "ndc")

## Horizontal line
pushViewport(viewport())
grid.lines(x = c(X1, X2), y = Y, gp = gpar(col = "red"))
popViewport()

最后，这里有一个几乎等价的、更普遍有用的方法.它使用了 Paul Murrell 在@mdsumner 的回答中链接的文章中演示的 grid.move.to() 和 grid.line.to() 函数:

Finally, here's an almost equivalent, and more generally useful approach. It employs the functions grid.move.to() and grid.line.to() demo'd by Paul Murrell in the article linked to in @mdsumner's answer:

library(grid)
library(gridBase)
par(mfrow=c(1,3))

barplot(VADeaths); vps1 <- do.call(vpStack, baseViewports())
barplot(VADeaths) 
barplot(VADeaths); vps3 <- do.call(vpStack, baseViewports())

pushViewport(vps1)
Y <- convertY(unit(50,"native"), "npc")
popViewport(3)

grid.move.to(x = unit(0, "npc"), y = Y, vp = vps1)
grid.line.to(x = unit(1, "npc"), y = Y, vp = vps3, 
             gp = gpar(col = "red"))

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