仅在R中将ggplot上的x轴显示为月

我在这里有一个非常简单的问题.我有一个2009-2012年的数据集.我想用多个方面绘制数据.我创建了如下的多面图.

I have a very simple question here. I have a dataset from 2009-2012. I want to plot the data with facets. I have created the faceted plot as follows.

R码

ggplot(al02428400,aes(x=date,y=as.numeric(Discharge)))+geom_line()+ylab("Discharge(cfs)")+facet_wrap(~Year,scales=("free_x"))+theme_bw()

上面的R代码的输出如下:

The output of the above R code is as follows:

在X轴上，我只想显示月份.默认情况下，它显示月份和年份.有什么办法可以摆脱一年的困扰?

On the X-axis I only want to show the month. By default it is showing month and year. Is there any way I can get rid of year ?

完全可复制的代码如下:

library(ggplot2)

url <- "http://nwis.waterdata.usgs.gov/usa/nwis/uv/?cb_00060=on&cb_00065=on&format=rdb&period=&begin_date=2009-01-01&end_date=2012-12-31&site_no=02428400"
download.file(url,destfile="Data load for stations/data/alabamariver-at-monroeville-2009.txt")

al02428400 <- read.table("Data load for stations/data/alabamariver-at-monroeville-2009.txt",header=T,skip=1,sep="\t")
head(al02428400)

sapply(al02428400,class)
al02428400 <- al02428400[-1,]

names(al02428400)<- c("Agency","SiteNo","Datetime", "TZ","Discharge","Status","Gageheight","gstatus")
al02428400$date <- strptime(al02428400$Datetime, format="%Y-%m-%d %H:%M")

al02428400$Discharge <- as.numeric(as.character(al02428400$Discharge))
al02428400$Year <- as.numeric(format(al02428400$date, "%Y"))
ggplot(al02428400,aes(x=date,y=as.numeric(Discharge)))+geom_line()+ylab("Discharge(cfs)")+facet_wrap(~Year,scales=("free_x"))+theme_bw()

谢谢.