Time Series Plot
Monthly sales with optional trend overlay — the starting point of any demand analysis
Seasonal Plot (gg_season style)
Each year overlaid on the same Jan–Dec axis — reveals consistent seasonal shape
Seasonal Subseries Plot (gg_subseries style)
Each month's values across years with mean line — shows month-by-month consistency
Time Series Decomposition
Additive: Y = Trend + Seasonal + Remainder — separating signal from noise
Seasonal Strength
FS = max(0, 1 − Var(Remainder) / Var(Seasonal + Remainder))
Autocorrelation Function (ACF)
Correlation at each lag — seasonal spikes at lag 12, 24 reveal annual patterns
Analysis & Supply Chain Implications
Based on Hyndman & Athanasopoulos, Forecasting: Principles and Practice, 3rd ed. (FPP3) & Wang, Smith & Hyndman (2006)