The file Natural Gas contains the U.S. monthly average commercial and residential price for natural gas in dollars per thousand cubic feet from January 2008 through December 2018.
For the commercial price and the residential price:
a. Construct a time-series plot.
b. What pattern, if any, is present in the data?
c. Construct a scatter plot of the commercial price and the residential price.
d. What conclusion can you reach about the relationship between the commercial price and the residential price?