I am describing a trend in monthly Immunization totals to hospital
management.  Immunizations are up 17% over last year's total at this time.  
However, a simple linear trendline points downward because of seasonal work
patterns - very high in the beginning, then lower, then gradually bending
upward in the last several months.  A moving average "trend"  merely
approximates this year's work pattern - so, why display it at all?  Yet, if I
were to choose to display a trendline,  a moving average approximates the
actual peformance of this department.  Is it useful to include a trendline in
this situation?   (In a similar vein, I use a moving average to trend
physician's monthly peformance, but the overall reaction has been "so what?").