2.5 Seasonal subseries plots
An alternative plot that emphasises the seasonal patterns is where the data for each season are collected together in separate mini time plots.
a10 |> gg_subseries(Cost) + labs( y = "$ (millions)", title = "Australian antidiabetic drug sales" )
The blue horizontal lines indicate the means for each month. This form of plot enables the underlying seasonal pattern to be seen clearly, and also shows the changes in seasonality over time. It is especially useful in identifying changes within particular seasons. In this example, the plot is not particularly revealing; but in some cases, this is the most useful way of viewing seasonal changes over time.
Example: Australian holiday tourism
Australian quarterly vacation data provides an interesting example of how these plots can reveal information. First we need to extract the relevant data from the
tourism tsibble. All the usual
tidyverse wrangling verbs apply. To get the total visitor nights spent on Holiday by State for each quarter (i.e., ignoring Regions) we can use the following code. Note that we do not have to explicitly group by the time index as this is required in a
holidays <- tourism |> filter(Purpose == "Holiday") |> group_by(State) |> summarise(Trips = sum(Trips))
holidays #> # A tsibble: 640 x 3 [1Q] #> # Key: State  #> State Quarter Trips #> <chr> <qtr> <dbl> #> 1 ACT 1998 Q1 196. #> 2 ACT 1998 Q2 127. #> 3 ACT 1998 Q3 111. #> 4 ACT 1998 Q4 170. #> 5 ACT 1999 Q1 108. #> 6 ACT 1999 Q2 125. #> 7 ACT 1999 Q3 178. #> 8 ACT 1999 Q4 218. #> 9 ACT 2000 Q1 158. #> 10 ACT 2000 Q2 155. #> # … with 630 more rows
Time plots of each series show that there is strong seasonality for most states, but that the seasonal peaks do not coincide.
autoplot(holidays, Trips) + labs(y = "Overnight trips ('000)", title = "Australian domestic holidays")
To see the timing of the seasonal peaks in each state, we can use a season plot. Figure 2.10 makes it clear that the southern states of Australia (Tasmania, Victoria and South Australia) have strongest tourism in Q1 (their summer), while the northern states (Queensland and the Northern Territory) have the strongest tourism in Q3 (their dry season).
gg_season(holidays, Trips) + labs(y = "Overnight trips ('000)", title = "Australian domestic holidays")
The corresponding subseries plots are shown in Figure 2.11.
holidays |> gg_subseries(Trips) + labs(y = "Overnight trips ('000)", title = "Australian domestic holidays")
This figure makes it evident that Western Australian tourism has jumped markedly in recent years, while Victorian tourism has increased in Q1 and Q4 but not in the middle of the year.