在文本中使用adj参数时,"大"标签的水平对齐会被抵消

Question

在text,该参数adj允许调整labels相对于x和y.例如,adj的值(0,1)是指左上对准,即左侧,标签的顶部拐角被放置在给定的x,y坐标.

这适用于默认字符扩展cex = 1.但是当我想要一个更大的标签,通过增加创建时cex,标签的位置从给定的坐标和adjustment 水平偏移.

这是一个小例子,它说明了这一点:

# a basic plot
plot(1:10, asp = 1, cex = 1)

# a red reference mark at 5, 5
points(x = 5, y = 5, pch = 3, cex = 3, col = 'red')

# a label of default size (cex = 1), left top adjusted
text(x = 5, y = 5, labels = 'H', cex = 1, adj = c(0, 1))

# a large label (cex = 8) with same position and adjustment as above becomes offset horizontally
text(x = 5, y = 5, labels = 'H', cex = 8, adj = c(0, 1), col = rgb(0.1, 0.9, 0.1, 0.5))

左/右底部/顶部对齐的所有组合都会发生水平偏移:

plot(1:10, cex = 1)
points(x = 5, y = 5, pch = 3, lwd = 4, cex = 4, col = 'red')

text(x = 5, y = 5, labels = "H", cex = 1, adj = c(0, 0))
text(x = 5, y = 5, labels = "H", cex = 1, adj = c(0, 1))
text(x = 5, y = 5, labels = "H", cex = 1, adj = c(1, 0))
text(x = 5, y = 5, labels = "H", cex = 1, adj = c(1, 1))

text(x = 5, y = 5, labels = "H", cex = 8, adj = c(0, 0), col = "green")
text(x = 5, y = 5, labels = "H", cex = 8, adj = c(0, 1), col = "green")
text(x = 5, y = 5, labels = "H", cex = 8, adj = c(1, 0), col = "green")
text(x = 5, y = 5, labels = "H", cex = 8, adj = c(1, 1), col = "green")

cex> 1 时如何避免标签的水平偏移？

Answer 1

这个问题可能有点难以解决。首先尝试解释原因，然后是潜在的解决方案。

正如 R 董事会成员 Brian Ripley 在此处的 R 帮助邮件列表中所写：

“ R 图形中的文本字符串直接以指定的字体绘制，而不是作为单个字母绘制”。

任何字体中的字母（或数字、标点符号和形状）都由字形表示。每个字形在两侧都有水平空间，即所谓的左右侧轴承。参见例如此处、此处 ('glyph metrics')和此处。

尽管使用cex = 1. 当您增加绘图中“字形”的大小时（使用cex），不仅字符本身的大小会增加，而且大小轴承的绝对宽度也会增加。

里普利因此得出结论：
“所以你对 R 中的字母间距无能为力。 ”

这个Q＆A显示了黑客，以减少空间之间的信件。不过，拆除前导左侧轴承可能会更棘手。

一个潜在的解决方案可能是使用systemfonts::shape_string抓住左轴承，然后相应地调整 x 位置。

这是一些带有坐标的字符串的示例。cex使用原始 x 值（浅灰色）和 x 值减去轴承（深灰色）绘制字符串（带有“大” ）。

d <- data.frame(x = 1:3, y = 1:3, labs = c("Where", "is", "Here"))

# set pointsize, cex and resolution
ps <- 12
cex <- 8
res <- 72

# calculate left bearing in pixels
left_bear_px <- shape_string(d$labs, size = ps * cex)$metrics$left_bearing

# open device
png("pp.png", width = 10, height = 5, units = "in", res = res)

# plot with "cross hair"
plot(x = d$x, y = d$y, pch = 3, cex = 3, col = "red", xlim = c(0, 5), ylim = c(0, 3))

# convert unit of bearing from pixel to xy: multiply by xy / pixel ratio
left_bear_xy <- left_bear_px * ((par("cxy") / par("cra"))[1])

# add text at original positions (light grey)
text(x = d$x, y = d$y, labels = d$labs,
     cex = cex, adj = c(0, 1), col = grey(0.6, alpha = 0.5))

# x values with left bearing removed (dark grey)
text(x = d$x - left_bear_xy, y = d$y, labels = d$labs,
     cex = cex, adj = c(0, 1), col = grey(0.1, alpha = 0.5))    

dev.off()