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aq1=read.csv(file="http://stateair.net/dos/historical/Hanoi/2015/Hanoi_PM2.5_2015_12_MTD.csv",header=TRUE)
aq2=read.csv(file="http://stateair.net/dos/historical/Hanoi/2016/Hanoi_PM2.5_2016_4_MTD.csv", header=TRUE)
aq3=read.csv(file="http://stateair.net/dos/historical/Hanoi/2016/Hanoi_PM2.5_2016_YTD.csv", header=TRUE)
aq1=subset(aq1,Value!=-999)
fit=lm(day ~ value)
fit
plot(aq1$Day,aq1$Value)
abline(v=2.5,col=3,lty=3)