How have emissions from motor vehicle sources changed from 1999-2008 in Baltimore City? The following plot will ansewr this question

main<- readRDS("summarySCC_PM25.rds")
SCC <- readRDS("Source_Classification_Code.rds")
Bal=main[main$fips=="24510",]
motor=SCC[SCC$Data.Category=="Onroad",]
Motor.Bal=Bal[Bal$SCC%in%motor$SCC,]
plot5=xtabs(Motor.Bal$Emissions~as.factor(Motor.Bal$year),data=Motor.Bal)
plot(plot5,type="l",xlab="Year",ylab = "Motor Vehicle Emissions")

Compare emissions from motor vehicle sources in Baltimore City with emissions from motor vehicle sources in Los Angeles County, California (fips == 06037). To see which city has seen greater changes over time in motor vehicle emissions, we can refer to the following plot:

## Warning: package 'ggplot2' was built under R version 3.2.2