Понял это, добавив поле счетчика во фрейм данных.
Новый код:
library(ggplot2)
# dataset
metric=c(rep("MAE" , 12) , rep("RMSE" , 12) )
horizon=rep(c("January 2017" , "February 2017" , "March 2017", "April 2017", "May 2017", "June 2017",
"July 2017", "August 2017", "September 2017", "October 2017", "November 2017", "December 2017") , 2)
Perc_Change=c(-24,-55,-40,NA,NA,-2,-22,-28,-12,-12,-2,-8,-15,-44,-37,NA,NA,3,-21,-28,-7,-15,3,-9)
count=rep(c(1,2,3,4,5,6,7,8,9,10,11,12),2)
data=data.frame(count,metric,horizon,Perc_Change)
# Faceting
ggplot(data, aes(y=Perc_Change, x=metric, fill=metric)) +
geom_bar( stat="identity") +
facet_wrap(count~horizon)+scale_fill_manual(values=c("darkblue", "darkred"))
, который производит ...
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