R ggplot dateTime data and use year as grouping variable

Question

I have some time series data, hourly, that runs across 5 years. I would like to plot my time series data with year as a grouping variable, so there are 5 lines across all dates and times of the year. Simply plotting dateTime on the x-axis and using lubridate::year(dateTime) as the grouping/colour in aes makes one long line with 5 different colours on it.

rough example data;

require(data.table)
require(lubridate)
require(ggplot2)

# this is just 7 days of hourly data, over 3 separate years
dt <- data.table(date = c(seq(as.Date("2018-03-01"), as.Date("2018-03-07"),by="day"), seq(as.Date("2019-03-01"), as.Date("2019-03-07"),by="day"), seq(as.Date("2020-03-01"), as.Date("2020-03-07"),by="day")), hr = rep(1:24, 21))
dt[, value := sin(hr)*sample(1:3,1)]

dt[, dateTime := as.POSIXct(paste0(date," ",hr,":00:00"), format="%Y-%m-%d %H:%M")]

# the result should be an x-axis of 7 days/hours, with three lines for years. 
# the below is obviously not that
ggplot(dt, aes(x=dateTime,y=value,group=year(dateTime), colour=year(dateTime)))+
  geom_line()

I thought there would be a way of formatting posix times as just month/day/time without the year component, but it appears to just return NA.

(ps not really interested in grouping by yday , for example, as i want the intricacies of the hourly cycles to be plotted)

Answer 1

You need a common timestamp for plotting on the x-axis.. so create one ( plotDate ) by setting all years in the dateTime to the year 2000 (or whatever...)
On creating the labels for the x-axis, just leave out the dummy-year value in the formatting.

# create some variables to use for plotting
dt[, year := lubridate::year(dateTime)]
dt[, datePlot := update(dateTime, year = 2000)]

#now plot
ggplot(data = dt, aes(x = datePlot, y = value, group = year, color = as.factor(year))) +
  geom_line(size = 1) + 
  scale_x_datetime(breaks = "12 hours", 
                   labels = function(x) format(x, "%d %b %H:%M")) +
  theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 1)) +
  labs(x = "dateTime", color = "year")