According to the U.S. Energy Information Administration, a heating degree-day is a relative measurement of outdoor temperature that indicates how far the average daily temperature is below 65° Fahrenheit. The higher the number of heating degree-days, the greater the national demand for heating energy from sources like natural gas and electricity. The following monthly data show the number of heating degree-days from 2005 through 2008.

a. Construct a graph of the time series. Does the overall trend appear to be upward or downward?
b. Determine the three-month and five-month centered moving average curves and superimpose each on the original time series.

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2005 859676 648 2006 687 73 600 264137 23 2007 84 853 502372 24 2008 892 741 617 319 183 26 2 2 305 186 25 2 2005 3 2006 2 2007 5 2008 5 236 466 866 82 304 467 690 175 521 800 281 534 831 2 13 52