plot1, plot2: Add code

- strptime and paste to treat date
- using read.csv.sql to subset data while reading input
plot3.R: Add code

- legend(.., lty=c(1,1,1)) to put lines instead of points (pch)
- The order of plot calls matter. The largest one should be
  called first.
plot4.R: Add code

- Initiate 4 graphs by par(mfcol=c(2,2))
- legend(.., bty="n") to remove the border line around legend
Modified code for PNG output; Added the PNG files