Adding Different Percentiles in boxplots in R

Question

I am failry new to R and recently used it to make some Boxplots. I also added the mean and standard deviation in my boxplot. I was wondering if i could add some kind of tick mar

Answer 1

Here is one way, using quantile() to compute the relevant percentiles for you. I add the marks using rug().

set.seed(1)
X <- rnorm(200)
boxplot(X, yaxt = "n")

## compute the required quantiles
qntl <- quantile(X, probs = c(0.85, 0.90))

## add them as a rgu plot to the left hand side
rug(qntl, side = 2, col = "blue", lwd = 2)

## add the box and axes
axis(2)
box()

Update: In response to the OP providing str() output, here is an example similar to the data that the OP has to hand:

set.seed(1) ## make reproducible
HWD <- data.frame(Hour = rep(0:23, 10),
                  Houston_Load = rnorm(24*10))

Now get I presume you want ticks at 85th and 90th percentiles for each Hour? If so we need to split the data by Hour and compute via quantile() as I showed earlier:

quants <- sapply(split(HWD$Houston_Load, list(HWD$Hour)),
                 quantile, probs = c(0.85, 0.9))

which gives:

R> quants <- sapply(split(HWD$Houston_Load, list(HWD$Hour)),
+                  quantile, probs = c(0.85, 0.9))
R> quants
            0         1        2         3         4         5        6
85% 0.3576510 0.8633506 1.581443 0.2264709 0.4164411 0.2864026 1.053742
90% 0.6116363 0.9273008 2.109248 0.4218297 0.5554147 0.4474140 1.366114
            7         8        9       10        11        12       13       14
85% 0.5352211 0.5175485 1.790593 1.394988 0.7280584 0.8578999 1.437778 1.087101
90% 0.8625322 0.5969672 1.830352 1.519262 0.9399476 1.1401877 1.763725 1.102516
           15        16        17        18       19        20       21
85% 0.6855288 0.4874499 0.5493679 0.9754414 1.095362 0.7936225 1.824002
90% 0.8737872 0.6121487 0.6078405 1.0990935 1.233637 0.9431199 2.175961
          22        23
85% 1.058648 0.6950166
90% 1.145783 0.8436541

Now we can draw marks at the x locations of the boxplots

boxplot(Houston_Load ~ Hour, data = HWD, axes = FALSE)
xlocs <- 1:24 ## where to draw marks
tickl <- 0.15 ## length of marks used
for(i in seq_len(ncol(quants))) {
    segments(x0 = rep(xlocs[i] - 0.15, 2), y0 = quants[, i],
             x1 = rep(xlocs[i] + 0.15, 2), y1 = quants[, i],
             col = c("red", "blue"), lwd = 2)
}
title(xlab = "Hour", ylab = "Houston Load")
axis(1, at = xlocs, labels = xlocs - 1)
axis(2)
box()
legend("bottomleft", legend = paste(c("0.85", "0.90"), "quantile"),
       bty = "n", lty = "solid", lwd = 2, col = c("red", "blue"))

The resulting figure should look like this: