# Transform a ggplot2 axis to a percentage scale

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When plotting a variable whose unit of measure is percent it’s best practice to have the axis labels contain the percentage sign (%). This makes it obvious to anyone looking at the data visualization that they are dealing with percentages.

To illustrate this let’s create an example dataset.

library(dplyr) data(mtcars) cyl <- mtcars %>% count(cyl) %>% mutate(pct = n / sum(n) * 100) %>% print() ## # A tibble: 3 x 3 ## cyl n pct #### 1 4 11 34.4 ## 2 6 7 21.9 ## 3 8 14 43.8

To create a bar chart displaying these data I will use my `ggcharts`

package which provides a high-level interface to produce plots using `ggplot2`

.

library(ggcharts) (p <- bar_chart(cyl, cyl, pct))

Next, let’s try to change the axis labels to include a percentage sign using the `percent()`

function from the `scales`

package.

p + scale_y_continuous(labels = scales::percent)

Something is not right here! 4000%!? That seems a bit excessive. The problem here is that by default `scales::percent()`

multiplies its input value by 100. This can be controlled by the `scale`

parameter.

scales::percent(100, scale = 1) ## [1] "100%"

However, `scale_y_continuous()`

expects a function as input for its `labels`

parameter not the actual labels itself. Thus, using `percent()`

is not an option anymore. Fortunately, the `scales`

package offers a function called `percent_format()`

that returns the `percent()`

function with changed defaults.

pct1 <- scales::percent_format(scale = 1) pct1(100) ## [1] "100%"

Passing this function to `labels`

produces the desired result.

p + scale_y_continuous(labels = scales::percent_format(scale = 1))

Alternatively, one can simply calculate a fraction instead of the actual percentage.

cyl2 <- mtcars %>% count(cyl) %>% mutate(pct = n / sum(n)) bar_chart(cyl2, cyl, pct) + scale_y_continuous(labels = scales::percent)

However, notice that suddenly all labels are printed with one decimal place. I think that’s undesireable given that the labels are all whole numbers.

To change this the `percent_format()`

function has a paramter called `accuracy`

.

bar_chart(cyl2, cyl, pct) + scale_y_continuous(labels = scales::percent_format(accuracy = 1))

To me all of this is confusing (to say the least). That’s why I decided to come up with a better solution. After all, it should be possible to determine `scale`

and `accuracy`

directly from the data, right?

My solution is the `scale_y_pct()`

function which is part of my `scalesextra`

package.

library(scalesextra) bar_chart(cyl, cyl, pct) + scale_y_pct()

bar_chart(cyl2, cyl, pct) + scale_y_pct()

As you can see, regardless of whether your data is a fraction of 1 or a true percentage the data is scaled correctly. Furthermore, in both cases no decimal is displayed as all labels are integers.

You can pass any parameter of `scale_y_continuous()`

to `scale_y_pct()`

, e.g. `breaks`

.

bar_chart(cyl, cyl, pct) + scale_y_pct(breaks = c(12.5, 30.75))

Notice that the number of decimal places displayed is consistent for all labels and automatically determined from the value with the highest number of decimal places. Again, this does not happen automatically when using `percent_format()`

.

bar_chart(cyl, cyl, pct) + scale_y_continuous( labels = scales::percent_format(scale = 1), breaks = c(12.5, 30.75) )

`scalesextra`

is in very early development and thus only available from GitHub. You can install it by running these commands in you `R`

console.

if (!"remotes" %in% installed.packages()) { install.packages("remotes") } remotes::install_github("thomas-neitmann/scalesextra")

Please test `scale_y_pct()`

(and its sister `scale_x_pct()`

) and let me know what you think in the comments. Should you find a bug (which is likely given the early stage of development), please open an issue on GitHub.

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