Normalized and Non-Normalized Diverging barchart

[spsanderson]

Compare variation in values between a small number of items (or categories) with respect to a fixed reference.

Diverging Bars is a bar chart that can handle both negative and positive values. This can be implemented by a smart tweak with geom_bar(). But the usage of geom_bar() can be quite confusing. That's because it can be used to make a bar chart as well as a histogram.

By default, geom_bar() has the stat set to count. That means, when you provide just a continuous X variable (and no Y variable), it tries to make a histogram out of the data.

In order to make a bar chart create bars instead of the histogram, you need to do two things.

Set stat=identity
Provide both x and y inside aes() where, x is either character or factor and y is numeric.
In order to make sure you get diverging bars instead of just bars, make sure, your categorical variable has 2 categories that change values at a certain threshold of the continuous variable. In the below example, the mpg from mtcars dataset is normalized by computing the z score. Those vehicles with mpg above zero are marked green and those below are marked red.

The following should be in the parameters list:

1. .data
2. .plot_title
3. .plot_subtitle
4. .plot_caption
5. .x_axis_title
6. .y_axis_ttle
7. main ggplot aes() x, y, label
8. .interactive - return plotly plot?

library(ggplot2)
theme_set(theme_bw())  

# Data Prep
data("mtcars")  # load data
mtcars$`car name` <- rownames(mtcars)  # create new column for car names
mtcars$mpg_z <- round((mtcars$mpg - mean(mtcars$mpg))/sd(mtcars$mpg), 2)  # compute normalized mpg
mtcars$mpg_type <- ifelse(mtcars$mpg_z < 0, "below", "above")  # above / below avg flag
mtcars <- mtcars[order(mtcars$mpg_z), ]  # sort
mtcars$`car name` <- factor(mtcars$`car name`, levels = mtcars$`car name`)  # convert to factor to retain sorted order in plot.

# Diverging Barcharts
ggplot(mtcars, aes(x=`car name`, y=mpg_z, label=mpg_z)) + 
  geom_bar(stat='identity', aes(fill=mpg_type), width=.5)  +
  scale_fill_manual(name="Mileage", 
                    labels = c("Above Average", "Below Average"), 
                    values = c("above"="#00ba38", "below"="#f8766d")) + 
  labs(subtitle="Normalised mileage from 'mtcars'", 
       title= "Diverging Bars") + 
  coord_flip()
ggplot2 Diverging Bars