替代矢量的Plotmath表达式

Question

我试图用10^n的值在每个第三个刻度处标记一个轴，格式化为上标。我正在使用grid图形。我在做一些粗心大意的事情。这里有一个MWE（或我看到有一些运动，称之为reprex）：

library("grid") 
grid.newpage() 
axisVP <- viewport(width = unit(6, "inches"), height = unit(0.01, "inches")) 
pushViewport(axisVP) 
labs <- c(1, "", "", expression(10^{-3}), "", "", expression(10^{-6}), "", "", 
    expression(10^{-9}), "", "", expression(10^{-12})) 
grid.xaxis(at = seq(0, 1, length.out = 15)[-c(1,15)], label = labs) 

不过，这仅与标签1.第一跳在str(labs)看它是与长度13的表达如预期。表达式被grid.xaxis接受。所以我不太确定为什么只显示第一个值。我已经在SO上探讨了相关的问题，但大多数似乎都将单个表达式作为轴标签或标题处理，而不是一系列表达式。大多数涉及轴标签的问题都试图使用专门的功能标记每个刻度。

Answer 1

这里就是我想要做的：

library(grid) 
grid.newpage() 
axisVP <- viewport(width = unit(6, "inches"), height = unit(0.01, "inches")) 
pushViewport(axisVP) 

## First plot an axis with all ticks and no labels 
ticks_at <- seq(1/15, 14/15, length.out=13) 
grid.xaxis(at = ticks_at, label=FALSE) 

## Then add labels where you want them 
labs <- parse(text=c("1", "10^{-3}", "10^{-6}", "10^{-9}", "10^{-12}")) 
## Alternatively, use this, though in practice it's more cumbersome 
# labs <- c(expression(1), expression(10^{-3}), expression(10^{-6}), 
#   expression(10^{-9}), expression(10^{-12})) 
labs_at <- seq(1/15, 14/15, length.out=5) 
grid.xaxis(at = labs_at, label = labs)