ggplot: Generate facet grid plot with multiple series - r

Question

I have following data frame:
Quarter x y p q
1 2001 8.714392 8.714621 3.3648435 3.3140090
2 2002 8.671171 8.671064 0.9282508 0.9034387
3 2003 8.688478 8.697413 6.2295996 8.4379698
4 2004 8.685339 8.686349 3.7520135 3.5278024
My goal is to generate a facet plot where x and y column in one plot in the facet and p,q together in another plot instead of 4 facets.
If I do following:
x.df.melt <- melt(x.df[,c('Quarter','x','y','p','q')],id.vars=1)
ggplot(x.df.melt, aes(Quarter, value, col=variable, group=1)) + geom_line()+
facet_grid(variable~., scale='free_y') +
scale_color_discrete(breaks=c('x','y','p','q'))
I all the four series in 4 different facets but how do I combine x,y to be one while p,q to be in another together. Preferable no legends.

Answer 1

One idea would be to create a new grouping variable:
x.df.melt$var <- ifelse(x.df.melt$variable == "x" | x.df.melt$variable == "y", "A", "B")
You can use it for facetting while using variable for grouping:
ggplot(x.df.melt, aes(Quarter, value, col=variable, group=variable)) + geom_line()+
facet_grid(var~., scale='free_y') +
scale_color_discrete(breaks=c('x','y','p','q'), guide = F)

Answer 2

I think beetroot's answer above is more elegant but I was working on the same problem and arrived at the same place a different way. I think it is interesting because I used a "double melt" (yum!) to line up the x,y/p,q pairs. Also, it demonstrates tidyr::gather instead of melt.
library(tidyr)
x.df<- data.frame(Year=2001:2004,
x=runif(4,8,9),y=runif(4,8,9),
p=runif(4,3,9),q=runif(4,3,9))
x.df.melt<-gather(x.df,"item","item_val",-Year,-p,-q) %>%
group_by(item,Year) %>%
gather("comparison","comp_val",-Year,-item,-item_val) %>%
filter((item=="x" & comparison=="p")|(item=="y" & comparison=="q"))
> x.df.melt
# A tibble: 8 x 5
# Groups: item, Year [8]
Year item item_val comparison comp_val
<int> <chr> <dbl> <chr> <dbl>
1 2001 x 8.400538 p 5.540549
2 2002 x 8.169680 p 5.750010
3 2003 x 8.065042 p 8.821890
4 2004 x 8.311194 p 7.714197
5 2001 y 8.449290 q 5.471225
6 2002 y 8.266304 q 7.014389
7 2003 y 8.146879 q 7.298253
8 2004 y 8.960238 q 5.342702
See below for the plotting statement.
One weakness of this approach (and beetroot's use of ifelse) is the filter statement quickly becomes unwieldy if you have a lot of pairs to compare. In my use case I was comparing mutual fund performances to a number of benchmark indices. Each fund has a different benchmark. I solved this by with a table of meta data that pairs the fund tickers with their respective benchmarks, then use left/right_join. In this case:
#create meta data
pair_data<-data.frame(item=c("x","y"),comparison=c("p","q"))
#create comparison name for each item name
x.df.melt2<-x.df %>% gather("item","item_val",-Year) %>%
left_join(pair_data)
#join comparison data alongside item data
x.df.melt2<-x.df.melt2 %>%
select(Year,item,item_val) %>%
rename(comparison=item,comp_val=item_val) %>%
right_join(x.df.melt2,by=c("Year","comparison")) %>%
na.omit() %>%
group_by(item,Year)
ggplot(x.df.melt2,aes(Year,item_val,color="item"))+geom_line()+
geom_line(aes(y=comp_val,color="comp"))+
guides(col = guide_legend(title = NULL))+
ylab("Value")+
facet_grid(~item)
Since there is no need for an new grouping variable we preserve the names of the reference items as labels for the facet plot.