替代矢量的Plotmath表達式

Question

我試圖用10^n的值在每個第三個刻度處標記一個軸，格式化爲上標。我正在使用grid圖形。我在做一些粗心大意的事情。這裏有一個MWE（或我看到有一些運動，稱之爲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) 

不過，這僅與標籤1.第一跳在str(labs)看它是與長度13的表達如預期。表達式被grid.xaxis接受。所以我不太確定爲什麼只顯示第一個值。我已經在SO上探討了相關的問題，但大多數似乎都將單個表達式作爲軸標籤或標題處理，而不是一系列表達式。大多數涉及軸標籤的問題都試圖使用專門的功能標記每個刻度。

Answer 1

這裏就是我想要做的：

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)