Formatting a scale_x_continuous axis with quarterly data - r

Question

I have a data set of counted things, in two groups, aggregated to quarterly counts. The Date_Qtr variable was derived from a larger data set with lubridate. The data frame is as follows.
dat = structure(list(Group = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L,
2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("TypeA",
"TypeB"), class = "factor"), Date_Qtr = c(2011.1, 2011.2, 2011.3,
2011.4, 2012.1, 2012.2, 2012.3, 2012.4, 2013.1, 2013.2, 2013.3,
2013.4, 2014.1, 2014.2, 2014.3, 2014.4, 2015.1, 2015.2, 2011.1,
2011.2, 2011.3, 2011.4, 2012.1, 2012.2, 2012.3, 2012.4, 2013.1,
2013.2, 2013.3, 2013.4, 2014.1, 2014.2, 2014.3, 2014.4, 2015.1,
2015.2), Counts = c(105L, 82L, 72L, 79L, 93L, 118L, 81L, 96L,
84L, 83L, 84L, 81L, 99L, 103L, 111L, 80L, 127L, 107L, 54L, 51L,
64L, 64L, 53L, 65L, 78L, 63L, 92L, 61L, 80L, 71L, 88L, 66L, 67L,
57L, 75L, 59L)), .Names = c("Group", "Date_Qtr", "Counts"), class = "data.frame", row.names = c(NA,
-36L))
I have plotted a time series in ggplot2 as follows, with the Date_Qtr variable as a scale_x_continuous. Formerly, when I plotted monthly data it was easy to assign breaks at quartely intervals.
ggplot(dat, aes(x = Date_Qtr, y = Counts)) +
geom_point( aes( color = Group ), size = 3) +
geom_line(aes(color = Group), size = 0.8) +
scale_y_continuous("Number of things",
limits = c(0, 150)) +
scale_x_continuous("Year and quarter when things were counted") +
theme_bw() +
theme(axis.text.x = element_text(angle = 45, vjust = 0.5),
legend.title = element_blank(),
legend.position = c(0.4, 0.85))
Is it possible, with a continuous scale, to represent the data as the actual quarters for each data point, preferably in a format "Jan-Mar 2012" etc.
Thanks in advance.

Answer 1

You could use Dates for the x-axis:
library(ggplot2)
library(scales)
library(zoo)
make_date <- function(x) {
year <- floor(x)
x <- year + (x - year)/0.4 - 0.125
as.Date(as.yearqtr(x))
}
format_quarters <- function(x) {
x <- as.yearqtr(x)
year <- as.integer(x)
quart <- as.integer(format(x, "%q"))
paste(c("Jan-Mar","Apr-Jun","Jul-Sep","Oct-Dec")[quart],
year)
}
ggplot(dat, aes(x = make_date(Date_Qtr), y = Counts)) +
geom_point( aes( color = Group ), size=3) +
geom_line(aes(color = Group), size=0.8) +
scale_y_continuous("Number of things",
limits=c(0,150)) +
scale_x_date("Year and quarter when things were counted",
breaks = date_breaks("3 months"),
labels = format_quarters) +
theme_bw() +
theme(axis.text.x = element_text(angle=45, vjust = 0.5),
legend.title=element_blank(),
legend.position = c(.4,0.85))

Answer 2

You can get the labels you want by adding a labels argument to scale_x_continuous.
Another issue is that Date_Qtr uses 0.1, 0.2, 0.3, and 0.4 for the quarters, so the quarters aren't numerically in the right location within each year on the x-axis. To fix this, I added a Date_Qtr_New column with the quarters spaced properly.
I also moved the axis titles to a separate labs statement, just to reduce clutter.
# Create new date-quarter values representing actual numerical distance in time
dat$Date_Qtr_New = floor(dat$Date_Qtr) + (as.numeric(gsub(".*\\.([1-4])","\\1", dat$Date_Qtr)) - 1) * 0.25
ggplot(dat, aes(x = Date_Qtr_New, y = Counts)) +
geom_point( aes( color = Group ), size=3) +
geom_line(aes(color = Group), size=0.8) +
scale_y_continuous(limits=c(0,150)) +
# Set quarterly breaks and use labels argument to get the labels we want
scale_x_continuous(breaks=seq(2011,2016.75,0.25),
labels=paste(c("Jan-Mar","Apr-Jun","Jul-Sep","Oct-Dec"),
rep(2011:2016,each=4))) +
labs(x="Year and quarter when things were counted",
y="Number of things") +
theme_bw() +
theme(axis.text.x = element_text(angle=45, vjust = 0.5),
legend.title=element_blank(),
legend.position = c(.4,0.85))