ggplot, facet, piechart: placing text in the middle of pie chart slices

Question

I'm trying to produce a facetted pie-chart with ggplot and facing problems with placing text in the middle of each slice:

dat = read.table(text = "Channel Volume Cnt                          AGENT   high   8344                          AGENT medium   5448                          AGENT    low  23823                          KIOSK   high  19275                          KIOSK medium  13554                          KIOSK    low  38293", header=TRUE)  vis = ggplot(data=dat, aes(x=factor(1), y=Cnt, fill=Volume)) +   geom_bar(stat="identity", position="fill") +   coord_polar(theta="y") +   facet_grid(Channel~.) +   geom_text(aes(x=factor(1), y=Cnt, label=Cnt, ymax=Cnt),              position=position_fill(width=1)) 

The output:

What parameters of geom_text should be adjusted in order to place numerical labels in the middle of piechart slices?

Related question is Pie plot getting its text on top of each other but it doesn't handle case with facet.

UPDATE: following Paul Hiemstra advice and approach in the question above I changed code as follows:

---> pie_text = dat$Cnt/2 + c(0,cumsum(dat$Cnt)[-length(dat$Cnt)])       vis = ggplot(data=dat, aes(x=factor(1), y=Cnt, fill=Volume)) +      geom_bar(stat="identity", position="fill") +      coord_polar(theta="y") +      facet_grid(Channel~.) +      geom_text(aes(x=factor(1),  --->               y=pie_text,                     label=Cnt, ymax=Cnt), position=position_fill(width=1)) 

As I expected tweaking text coordiantes is absolute but it needs be within facet data:

Answer 1

NEW ANSWER: With the introduction of ggplot2 v2.2.0, position_stack() can be used to position the labels without the need to calculate a position variable first. The following code will give you the same result as the old answer:

ggplot(data = dat, aes(x = "", y = Cnt, fill = Volume)) +    geom_bar(stat = "identity") +   geom_text(aes(label = Cnt), position = position_stack(vjust = 0.5)) +   coord_polar(theta = "y") +   facet_grid(Channel ~ ., scales = "free") 

To remove "hollow" center, adapt the code to:

ggplot(data = dat, aes(x = 0, y = Cnt, fill = Volume)) +    geom_bar(stat = "identity") +   geom_text(aes(label = Cnt), position = position_stack(vjust = 0.5)) +   scale_x_continuous(expand = c(0,0)) +   coord_polar(theta = "y") +   facet_grid(Channel ~ ., scales = "free") 

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OLD ANSWER: The solution to this problem is creating a position variable, which can be done quite easily with base R or with the data.table, plyr or dplyr packages:

Step 1: Creating the position variable for each Channel

# with base R dat$pos <- with(dat, ave(Cnt, Channel, FUN = function(x) cumsum(x) - 0.5*x))  # with the data.table package library(data.table) setDT(dat) dat <- dat[, pos:=cumsum(Cnt)-0.5*Cnt, by="Channel"]  # with the plyr package library(plyr) dat <- ddply(dat, .(Channel), transform, pos=cumsum(Cnt)-0.5*Cnt)  # with the dplyr package library(dplyr) dat <- dat %>% group_by(Channel) %>% mutate(pos=cumsum(Cnt)-0.5*Cnt) 

Step 2: Creating the facetted plot

library(ggplot2) ggplot(data = dat) +    geom_bar(aes(x = "", y = Cnt, fill = Volume), stat = "identity") +   geom_text(aes(x = "", y = pos, label = Cnt)) +   coord_polar(theta = "y") +   facet_grid(Channel ~ ., scales = "free")  

The result:

Answer 2

I would like to speak out against the conventional way of making pies in ggplot2, which is to draw a stacked barplot in polar coordinates. While I appreciate the mathematical elegance of that approach, it does cause all sorts of headaches when the plot doesn't look quite the way it's supposed to. In particular, precisely adjusting the size of the pie can be difficult. (If you don't know what I mean, try to make a pie chart that extends all the way to the edge of the plot panel.)

I prefer drawing pies in a normal cartesian coordinate system, using geom_arc_bar() from ggforce. It requires a little bit of extra work on the front end, because we have to calculate angles ourselves, but that's easy and the level of control we get as a result is more than worth it. I've used this approach in previous answers here and here.

The data (from the question):

dat = read.table(text = "Channel Volume Cnt AGENT   high   8344 AGENT medium   5448 AGENT    low  23823 KIOSK   high  19275 KIOSK medium  13554 KIOSK    low  38293", header=TRUE) 

The pie-drawing code:

library(ggplot2) library(ggforce) library(dplyr)  # calculate the start and end angles for each pie dat_pies <- left_join(dat,                       dat %>%                          group_by(Channel) %>%                         summarize(Cnt_total = sum(Cnt))) %>%   group_by(Channel) %>%   mutate(end_angle = 2*pi*cumsum(Cnt)/Cnt_total,      # ending angle for each pie slice          start_angle = lag(end_angle, default = 0),   # starting angle for each pie slice          mid_angle = 0.5*(start_angle + end_angle))   # middle of each pie slice, for the text label  rpie = 1 # pie radius rlabel = 0.6 * rpie # radius of the labels; a number slightly larger than 0.5 seems to work better,                     # but 0.5 would place it exactly in the middle as the question asks for.  # draw the pies ggplot(dat_pies) +    geom_arc_bar(aes(x0 = 0, y0 = 0, r0 = 0, r = rpie,                    start = start_angle, end = end_angle, fill = Volume)) +   geom_text(aes(x = rlabel*sin(mid_angle), y = rlabel*cos(mid_angle), label = Cnt),             hjust = 0.5, vjust = 0.5) +   coord_fixed() +   scale_x_continuous(limits = c(-1, 1), name = "", breaks = NULL, labels = NULL) +   scale_y_continuous(limits = c(-1, 1), name = "", breaks = NULL, labels = NULL) +   facet_grid(Channel~.) 

To show why I think this this approach is so much more powerful than the conventional (coord_polar()) approach, let's say we want the labels on the outside of the pie rather than inside. This creates a couple of problems, such as we will have to adjust hjust and vjust depending on the side of the pie a label falls, and also we will have to make the plot panel wider than high to make space for the labels on the side without generating excessive space above and below. Solving these problems in the polar coordinate approach is not fun, but it's trivial in the cartesian coordinates:

# generate hjust and vjust settings depending on the quadrant into which each # label falls dat_pies <- mutate(dat_pies,                    hjust = ifelse(mid_angle>pi, 1, 0),                    vjust = ifelse(mid_angle<pi/2 | mid_angle>3*pi/2, 0, 1))  rlabel = 1.05 * rpie # now we place labels outside of the pies  ggplot(dat_pies) +    geom_arc_bar(aes(x0 = 0, y0 = 0, r0 = 0, r = rpie,                    start = start_angle, end = end_angle, fill = Volume)) +   geom_text(aes(x = rlabel*sin(mid_angle), y = rlabel*cos(mid_angle), label = Cnt,                 hjust = hjust, vjust = vjust)) +   coord_fixed() +   scale_x_continuous(limits = c(-1.5, 1.4), name = "", breaks = NULL, labels = NULL) +   scale_y_continuous(limits = c(-1, 1), name = "", breaks = NULL, labels = NULL) +   facet_grid(Channel~.)