Render Plot 2 and save as plot2.png
Fixed some incorrect comments.
Render Plot 3 and save as plot3.png
Revert "Render Plot 3 and save as plot3.png"

This reverts commit 0b9d761b3c54ae44f8a6b1fb3dd32ab82bac48af.
Accidentally wrote plot3.R in plot2.R...
Properly render Plot 3 to plot3.png
Plots 1-3 are drawn by functions. Useful for plot4.R.
Delete unused code in plot1.R.
Render Plot 4 and save as plot4.png.