ggplot2: plotting several boxes using a loop

Question

I'm trying to draw a series of boxes in ggplot2 using a loop. Below, I have included a toy example.

# Load necessary library
library(ggplot2)

# Height of rectangle
box.size <- 0.5

# Colours for rectangles
my.cols <- c("red", "blue", "purple", "yellow", "green")

# Initialise plot & set limits
p <- ggplot() + xlim(0, 1) + ylim(0, 2.5)

# Loop through and draw boxes
for(i in 1:5){
  # Draw boxes
  p <- p + geom_rect(aes(xmin = 0, xmax = 1, ymin = (i - 1) * box.size, ymax = i * box.size), 
                 fill = my.cols[i])

  # Check that the loop is working
  print(i) 
}

# Plot graph
print(p)

This code only plots the final rectangle, but I can't figure out what I am doing wrong. The loop is running correctly because I included a print statement to check. Can someone point out my error and offer a solution, please?

ggplot is designed for plotting data. I'd recommend you construct a data frame (in a similar loop, if you want) and then plot that. As-is, you're trying to use a screwdriver as a hammer. — Gregor Thomas
– Gregor Thomas, Commented Jan 25, 2017 at 20:38
Okay, understood. Thanks for the helpful reply. Out of curiosity, though: why does the code fail? — Dan
– Dan, Commented Jan 26, 2017 at 0:14
I'm not sure exactly. My ggplot is strongly designed around the data frame. You almost certainly shouldn't be using aes() without a data frame, but even without that your code doesn't work. And the for loop has nothing to do with it - if you write out the first couple iterations by hand, the first layer works as expected and the second doesn't. — Gregor Thomas
– Gregor Thomas, Commented Jan 26, 2017 at 0:33

Jake Kaupp · Accepted Answer · 2017-01-25 20:46:26Z

4

I agree with Gregor. Just make a function, loop, or statement to construct the underlying data, then plot it with ggplot2.

library(ggplot2)

box.size <- 0.5

df <- data.frame(xmin = rep(0, 5),
           xmax = rep(1,5),
           ymin = (seq(1:5)-1) * box.size,
           ymax = seq(1:5) * box.size,
           fill = c("red", "blue", "purple", "yellow", "green"))

 ggplot(df) +
  geom_rect(aes(xmin = xmin, xmax = xmax, ymin = ymin, ymax = ymax, fill = fill)) +
  scale_fill_identity()

answered Jan 25, 2017 at 20:46

Jake Kaupp

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Phil · Accepted Answer · 2017-01-25 20:46:49Z

0

As noted by Gregor, ggplot is not meant to be used with loops. You need to set up your coordinates into a dataframe first:

library(dplyr)
library(ggplot2)
mydf <- data.frame(my.cols, tmp = rep(box.size, 5), i = 1:5)
mydf <- mutate(mydf, ymin = (i - 1) * tmp, ymax = tmp * i)
mydf <- select(mydf, -tmp, -i)

ggplot(mydf) + xlim(0,1) + ylim(0, 2.5) + geom_rect(aes(xmin = 0, xmax = 1, ymin = ymin, ymax = ymax), fill = my.cols)

answered Jan 25, 2017 at 20:46

Phil

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