year = 1990:1996                       # Create a series of years
x1 = c(10, 15, 14, 7, 14, 10, 18)      # Create some observation values for x1
x1ma = filter(x1,                      # Create a 4 period moving average for x1
  filter = c(.25, .25, .25, .25),      # Equal weights for the 4 periods
  sides = 1)                           # Backwards looking moving avg.

x1ma                                   # Display list output
as.double(x1ma)                        # Display as forced to numeric

# Plot of moving averate data --------------------------------------------------

png(filename = "illustrations/fig-9-2-moving avg-BW.png",
  units = "in",                        # Set measurements in inches
  res = 1200,                          # Set resolution at 1200dpi
  width = 6,                           # Width at 6 inches
  height = 4)                          # Height at 4 inches

par(mai = c(1, 1, .25, .25))           # Set margins for no title

plot(year, x1,                         # Plot x1 against year
  ylim = c(0, 20),                     # Set y axis scale
  type = "o",                          # Set as line plot with points
  pch = 16)                            # Use solid point style

points(year, x1ma,                     # Add x1 moving avg (x1ma) to plot
  type = "o",                          # Set type as line with points
  col = "darkgray",                    # Set color to dark gray
  pch = 16)                            # Use solid rather than open points

legend(x = 1996, y = 5,                # Location of legend
  xjust = 1,                           # Right justify legend box
  legend = 
    c("x1", "4 Period Moving Avg"),    # Legend Text
  col = c("black", "darkgray"),        # Legend Element Colors
  pch = 16)                            # Legend Element Styles

dev.off()                              # Output png file