Plot separate years on a common day-month scale

Question

I want to create a time series plot of temperatures for the summers of 2012 and 2013.

The only problem is that I want the data series to plot one on top of the other so they can be easily compared instead of sequentially along the date axis.

temp <- c(22, 22, 26, 23, 18, 20, 18, 17)
date <- as.Date(c("2012-06-01", "2012-07-01","2012-08-01","2012-09-01","2013-06-01","2013-07-01","2013-08-01","2013-09-01"))
year <- as.factor(c("2012", "2012", "2012", "2012","2013", "2013","2013","2013"))

df<- data.frame(temp, date, year)

Here's what I have so far using ggplot2

require(ggplot2)
ggplot(df, aes(date, temp, color=year))+
  geom_point()

The graph doesn't need to have the full dates listed on they x axis, in fact, it should probably just have month and day and that might solve the problem, i.e.

df$dayMo <- c("07-01", "07-02","07-03","07-04","07-01","07-02","07-03","07-04")

I didn't see a way to get as.Date or as.POSIXct (strptime) to allow this day-month format.

I'm also open to some other creative way of getting this done. Any ideas?

Answer 1

If your base dataset is temp and date, then this avoids manipulating the original data frame:

ggplot(df) +
  geom_point(aes(x=strftime(date,format="%m-%d"),
                 y=temp, 
                 color=strftime(date,format="%Y")), size=3)+
  scale_color_discrete(name="Year")+
  labs(x="date")

EDIT (Response to OP's comment).

So this combines the approach above with Henrik's, using dates instead of char for the x-axis, and avoiding modification of the original df.

library(ggplot2)
ggplot(df) +
  geom_point(aes(x=as.Date(paste(2014,strftime(date,format="%m-%d"),sep="-")),
                 y=temp, 
                 color=strftime(date,format="%Y")), size=3)+
  scale_color_discrete(name="Year")+
  labs(x="date")

Answer 2

Another possibility is to create a fake date where a single year is concatenated with month and day from the original 'date'. An x variable of class Date is easy to format with scale_x_date. Load scales package to access nice breaks and formatting functions: labels = date_format(); breaks = date_breaks(). See strptime for other date formats.

library(scales)

df$date2 <- as.Date(paste(2014, format(date, "%m-%d"), sep = "-"))

ggplot(df, aes(date2, temp, color = year)) +
  geom_point() +
  scale_x_date(labels = date_format("%m/%d"))

ggplot(df, aes(date2, temp, color=year)) +
  geom_point() +
  scale_x_date(labels = date_format("%b-%d"),
               breaks = date_breaks("months"))

Answer 3

If you want to plot both years in the same plot above each other then use dayMo as your x values (they don't have to be date object).

df$dayMo<-c("06-01", "07-01","08-01","09-01","06-01", "07-01","08-01","09-01")
ggplot(df, aes(dayMo, temp, color=year))+
  geom_point()