How to use multiple columns as the frame of an animation in gganimate package (R)?

Question

I am trying to use gganimate package of R, to create an animation of a bunch of histograms, where each frame of the animation shows a histogram of an image. I have ~400 images, so ~400 columns. My data looks like this:

| bins.left | bins.right | hist1 | hist 2 | ... | hist n |

and as you see, I need each column be considered as the Y value of the histogram, in each frame. In other words, my animation should iterate over the columns.

But all the examples that I have studied on the Internet, seem to be considering only one column as the identifier of the frames. For instance in this example:

mapping <- aes(x = gdpPercap, y = lifeExp, 
           size = pop, color = continent,
           frame = year) 
p <- ggplot(gapminder, mapping = mapping) +
  geom_point() +
  scale_x_log10()

the attribute 'Year' is considered as the iterator. This data looks like this:

  country continent  year lifeExp      pop gdpPercap
   <fctr>    <fctr> <int>   <dbl>    <int>     <dbl>
1 Afghanistan      Asia  1952  28.801  8425333  779.4453
2 Afghanistan      Asia  1957  30.332  9240934  820.8530
3 Afghanistan      Asia  1962  31.997 10267083  853.1007
4 Afghanistan      Asia  1967  34.020 11537966  836.1971
5 Afghanistan      Asia  1972  36.088 13079460  739.9811
6 Afghanistan      Asia  1977  38.438 14880372  786.1134

The reason that I don't want to modify my data to fit into such a pattern is that if I keep all the histograms in one column, my data frame will be extremely lengthy (length = ~ 16000 * 400) and difficult to handle. In addition, it is not intuitive to keep my data in such a confusing fashion. I believe there must be an easy solution to my problem. Any suggestion is highly appreciated.

Answer 1

As @Marius said, you can make this work if your data is in long format. Below I create some fake data and then make the animated plot.

library(tidyverse)
theme_set(theme_classic())
library(gganimate)

Here's fake data with 10 columns of values we want to turn into histograms.

set.seed(2)
dat = replicate(10, runif(100)) %>% as.data.frame

The data is in wide format, so first we'll convert it to long format with the gather function:

d = dat %>% gather(key, value)

In the new long format, the key column tells us which histogram column the data originally came from. We'll use this as our frame and run geom_histogram:

p = ggplot(d, aes(value, frame=key)) +
  geom_histogram() 

gganimate(p)

You can see that this is not what we want. ggplot actually generated a single histogram from all the data and the animation just shows us in succession the part of each stack that came from each value of key.

We need a way to get ggplot to create separate histograms and animate them. We can do that by pre-binning the data and using geom_rect to create the histogram bars:

d = dat %>% gather(key, value) %>% 
  mutate(bins = cut(value, breaks=seq(0,1,0.1), 
                    labels=seq(0,0.9,0.1) + 0.05, include.lowest=TRUE),
         bins = as.numeric(as.character(bins))) %>% 
  group_by(key, bins) %>% 
  tally

p = ggplot(d, aes(xmin=bins - 0.048, xmax=bins + 0.048, ymin=0, ymax=n, frame=key)) +
  geom_rect() +
  scale_y_continuous(limits=c(0, max(d$n)))

gganimate(p)

In response to your comment, I don't think you can use gganimate with data in wide format. gganimate requires a single frame column, which requires data in long format. However, gganimate is a wrapper around the animation package, and you can create an animated GIF file directly with a for loop and the saveGIF function:

library(animation)

saveGIF({
  for (i in names(dat)) {
    p = ggplot(dat, aes_string(i)) +
      geom_histogram(breaks=seq(0,1,0.1))
    print(p)
  }
}, movie.name="test.gif")