在直方图中创建辅助 y 轴

Question

I have found multiple ways to create a secondary y-axis in plot but I couldn't find a way to create a secondary y-axis in histogram.

Here is a sample code:

a <- sample(90:110, 50, replace=TRUE)
b <- runif(50, min=0, max=1)
hist(a)
lines(b)

b is too small to show in hist(a) so is there any way that I can see both in the histogram?

Answer 1

Technically a solution may be quite an identical to the approach proposed for the plots in this answer . The idea is to use overlapping of two plots as proposed by @r2evans.

It makes sense to use color coding:

# set color rules
col_a <- "red"
col_b <- "darkblue"
col_common <- "black"

Then let's draw the histogram and the plot:

# draw a histogram first
par(mar = c(5, 5, 5, 5) + 0.3)
hist(a, col = col_a, axes = FALSE, xlab = "", ylab = "", main = "")
# add both axes with the labels
axis(side = 1, xlim = seq(along.with = b), col = col_a, col.axis = col_a)
mtext(side = 1, text = "a_value", col = col_a, line = 2.5)
axis(side = 2, col = col_a, col.axis = col_a, ylab = "")
mtext(side = 2, text = "a_Frequency", col = col_a, line = 2.5)
# ... and add an overlaying plot
par(new=TRUE)
plot(b, ylim = c(0, 1), axes = FALSE, col = col_b, type = "l", xlab = "", ylab = "")
points(b, col = col_b, pch = 20, xlab = "", ylab = "")
axis(side = 3, xlim = seq(along.with = b), col = col_b, col.axis = col_b)
mtext(side = 3, text = "b_index", col = col_b, line = 2.5)
axis(side = 4, ylim = c(0, 1), col = col_b, col.axis = col_b)
mtext(side = 4, text = "b_value", col = col_b, line = 2.5)
box(col = col_common)