Shading every other vertical month, alternating white and gray - r

Question

I have boxplots of data with one set of boxplots for each month. But together, they get busy and it isn't clear which box goes with which date. Is there a way to shade every other vertical month light gray so I can easily see which ones go with which month?
Edit: I'm already using geom_polygon for another part of the plot that I commented out for now.
date <- seq(as.Date('2015-09-15'), as.Date('2016-09-30'), by = "2 days")
x <- rnorm(length(date))
date <- date[order(x)]
Date <- format(as.Date(date), "%Y-%m")
values <- rnorm(length(x),.16,.01)
type <- c(rep("a",63),rep("b",64),rep("c",64))
new.table <- as.data.frame.matrix(table(Date,type))
dataset <- data.frame(values,date,type,Date)
if(length(which(levels(factor(type))=="c"))==0){
count.data <- rep(0,length(levels(factor(Date))))
}else{count.data <- new.table[,names(new.table)=="c"]}
ly <- length(count.data)
max.count <- max(count.data)
max.right <- max.count*4
max.box <- max(dataset$values,na.rm=T)
min.box <- 5/4*min(dataset$values,na.rm=T)-max.box/4
box.25 <- (max.box-min.box)/4
x <- c(0:(ly+1),c((ly+1):0))
y <- c(0,count.data,rep(0,(ly+3)))*(box.25/max.count)+min.box
poly.data <- data.frame(x,y)
dates1 <- levels(factor(dataset$Date))
noB <- length(dates1)
new.table$Date <- rownames(new.table)
library(tidyverse)
library(gridExtra)
library(ggthemes)
library(ggplot2)
p <- ggplot(dataset,aes(x=Date,y=values,fill=type))+
geom_boxplot(position=position_dodge(width = 0.7))+
stat_boxplot(geom="errorbar",width=0.7)+
coord_cartesian(ylim = c(min.box,max.box))+
#geom_polygon(data=poly.data,mapping = aes(x=x,y=y),fill="grey30")+
#scale_y_continuous(sec.axis = sec_axis(~(.-min.box)*max.count/box.25, name = "Sec axis"),breaks = scales::pretty_breaks(n = 10))+
labs(title="Boxplot of values Over Time",y="values",x="Date (year-month)")+
theme_classic(base_size=15)+
theme(axis.text.x = element_text(angle = ifelse(noB>15,45,0), hjust=ifelse(noB>15,1,0.5)),panel.grid.major=element_line("light grey"))
p

Answer 1

I would use geom_rect with a separate data.frame (here: shades)
shades <- data.frame(xmin=seq(1.5,length(unique(dataset$Date))-1.5, 2),
xmax=seq(2.5,length(unique(dataset$Date))+.5, 2),
ymin=0, ymax=Inf)
ggplot(dataset,aes(x=Date,y=values,fill=type))+
geom_boxplot(position=position_dodge(width = 0.7))+
stat_boxplot(geom="errorbar",width=0.7)+
coord_cartesian(ylim = c(min.box,max.box))+
#geom_polygon(data=poly.data,mapping = aes(x=x,y=y),fill="grey30")+
#scale_y_continuous(sec.axis = sec_axis(~(.-min.box)*max.count/box.25, name = "Sec axis"),breaks = scales::pretty_breaks(n = 10))+
labs(title="Boxplot of values Over Time",y="values",x="Date (year-month)")+
theme_classic(base_size=15)+
theme(axis.text.x = element_text(angle = ifelse(noB>15,45,0), hjust=ifelse(noB>15,1,0.5)),panel.grid.major=element_line("light grey")) +
geom_rect(inherit.aes = F, data = shades, mapping = aes(xmin=xmin, xmax=xmax, ymin = ymin, ymax = ymax), alpha = 0.2)