vignettes/ggally_stats.Rmd
ggally_stats.Rmd
GGally
proposes several additional statistics that could be used with ggplot2
. As reminder, a statistic is always used in conjunction with a geometry. You can call a statistic from a geom_*()
or call a geometry from a stat_*()
. A statistic will compute new variables from the provided data
. These new variables could be mapped to an aesthetic using ggplot2::after_stat()
.
library(GGally, quietly = TRUE) #> Registered S3 method overwritten by 'GGally': #> method from #> +.gg ggplot2
stat_cross()
This statistic is intended to be used with two discrete variables mapped to x and y aesthetics. It will compute several statistics of a cross-tabulated table using broom::tidy.test()
and stats::chisq.test()
. More precisely, the computed variables are:
By default, stat_cross()
is using ggplot2::geom_points()
. If you can to plot the number of observations, you need to map after_stat(observed)
to an aesthetic (here size):
d <- as.data.frame(Titanic) ggplot(d) + aes(x = Class, y = Survived, weight = Freq, size = after_stat(observed)) + stat_cross() + scale_size_area(max_size = 20)
Note that the weight aesthetic is taken into account by stat_cross()
.
We can go further using a custom shape and filling points with standardized residual to identify visually cells who are over- or underrepresented.
ggplot(d) + aes(x = Class, y = Survived, weight = Freq, size = after_stat(observed), fill = after_stat(stdres)) + stat_cross(shape = 22) + scale_fill_steps2(breaks = c(-3, -2, 2, 3), show.limits = TRUE) + scale_size_area(max_size = 20)
We can easily recreate a cross-tabulated table.
ggplot(d) + aes(x = Class, y = Survived, weight = Freq) + geom_tile(fill = "white", colour = "black") + geom_text(stat = "cross", mapping = aes(label = after_stat(observed))) + theme_minimal()
Even more complicated, we want to produce a table showing column proportions and where cells are filled with standardized residuals. Note that stat_cross()
could be used with facets. In that case, computation is done separately in each facet.
ggplot(d) + aes( x = Class, y = Survived, weight = Freq, label = scales::percent(after_stat(col.prop), accuracy = .1), fill = after_stat(stdres) ) + stat_cross(shape = 22, size = 30) + geom_text(stat = "cross") + scale_fill_steps2(breaks = c(-3, -2, 2, 3), show.limits = TRUE) + facet_grid(rows = vars(Sex)) + labs(fill = "Standardized residuals") + theme_minimal()
stat_prop()
stat_prop()
is a variation of ggplot2::stat_count()
allowing to compute custom proportions according to the by aesthetic defining the denominator (i.e. all proportions for a same value of by will sum to 1). The by aesthetic should be a factor. Therefore, stat_prop()
requires the by aesthetic and this by aesthetic should be a factor.
When using position = "fill"
with geom_bar()
, you can produce a percent stacked bar plot. However, the proportions corresponding to the y axis are not directly accessible using only ggplot2
. With stat_prop()
, you can easily add them on the plot.
In the following example, we indicated stat = "prop"
to ggplot2::geom_text()
to use stat_prop()
, we defined the by aesthetic (here we want to compute the proportions separately for each value of x), and we also used ggplot2::position_fill()
when calling ggplot2::geom_text()
.
d <- as.data.frame(Titanic) p <- ggplot(d) + aes(x = Class, fill = Survived, weight = Freq, by = Class) + geom_bar(position = "fill") + geom_text(stat = "prop", position = position_fill(.5)) p
Note that stat_prop()
has properly taken into account the weight aesthetic.
stat_prop()
is also compatible with faceting. In that case, proportions are computed separately in each facet.
p + facet_grid(cols = vars(Sex))
If you want to display proportions of the total, simply map the by aesthetic to 1
. Here an example using a stacked bar chart.
ggplot(d) + aes(x = Class, fill = Survived, weight = Freq, by = 1) + geom_bar() + geom_text( aes(label = scales::percent(after_stat(prop), accuracy = 1)), stat = "prop", position = position_stack(.5) )
A dodged bar plot could be used to compare two distributions.
On the previous graph, it is difficult to see if first class is over- or under-represented among women, due to the fact they were much more men on the boat. stat_prop()
could be used to adjust the graph by displaying instead the proportion within each category (i.e. here the proportion by sex).
ggplot(d) + aes(x = Class, fill = Sex, weight = Freq, by = Sex, y = after_stat(prop)) + geom_bar(stat = "prop", position = "dodge") + scale_y_continuous(labels = scales::percent)
The same example with labels:
ggplot(d) + aes(x = Class, fill = Sex, weight = Freq, by = Sex, y = after_stat(prop)) + geom_bar(stat = "prop", position = "dodge") + scale_y_continuous(labels = scales::percent) + geom_text( mapping = aes( label = scales::percent(after_stat(prop), accuracy = .1), y = after_stat(0.01) ), vjust = "bottom", position = position_dodge(.9), stat = "prop" )
stat_weighted_mean()
stat_weighted_mean()
computes mean value of y (taking into account any weight aesthetic if provided) for each value of x. More precisely, it will return a new data frame with one line per unique value of x with the following new variables:
Let’s take an example. The following plot shows all tips received according to the day of the week.
data(tips, package = "reshape") ggplot(tips) + aes(x = day, y = tip) + geom_point()
To plot their mean value per day, simply use stat_weighted_mean()
.
ggplot(tips) + aes(x = day, y = tip) + stat_weighted_mean()
We can specify the geometry we want using geom
argument. Note that for lines, we need to specify the group aesthetic as well.
ggplot(tips) + aes(x = day, y = tip, group = 1) + stat_weighted_mean(geom = "line")
An alternative is to specify the statistic in ggplot2::geom_line()
.
Of course, it could be use with other geometries. Here a bar plot.
p <- ggplot(tips) + aes(x = day, y = tip, fill = sex) + stat_weighted_mean(geom = "bar", position = "dodge") + ylab("mean tip") p
It is very easy to add facets. In that case, computation will be done separately for each facet.
p + facet_grid(rows = vars(smoker))
stat_weighted_mean()
could be also used for computing proportions as a proportion is technically a mean of binary values (0 or 1).
ggplot(tips) + aes(x = day, y = as.integer(smoker == "Yes"), fill = sex) + stat_weighted_mean(geom = "bar", position = "dodge") + scale_y_continuous(labels = scales::percent) + ylab("proportion of smoker")
Finally, you can use the weight aesthetic to indicate weights to take into account for computing means / proportions.
d <- as.data.frame(Titanic) ggplot(d) + aes(x = Class, y = as.integer(Survived == "Yes"), weight = Freq, fill = Sex) + geom_bar(stat = "weighted_mean", position = "dodge") + scale_y_continuous(labels = scales::percent) + labs(y = "Proportion who survived")