R 如何对齐不同高度的轴标签？

如何水平对齐不同高度的（x轴）标签？ 以下是一个例子：

nms <- c(expression(A), expression(B), expression(C),
         expression(D[1]), expression(E^1), expression(F))
boxplot(count ~ spray, data=InsectSprays, names=nms)

---

nms例如，您可以使用
mtext
手动绘制轴

## reset defalt names 
boxplot(count ~ spray, data=InsectSprays,names=NA)
## adjustemnt vectors , see that only d_1 has an adj !=0
adj=c(0,0,0,0.2,0,0)
## loop over expressions and plot them one by one
for(s in seq_along(nms))
  mtext(nms[s], side=1, line=1,at=s,padj=adj[s])