Omit the middle year (2003), average each half, plot the two semi-averages at the mid-years (2001, 2005), and join them: the trend rises 2.5 units per year.

Method of semi-averages. There are 7 years (2000–2006), an odd number, so the middle year (2003) is omitted and the data split into two equal halves of 3 years each.

Step 1 — first-half average (2000, 2001, 2002).

$\bar{y}_1 = \frac{105 + 115 + 120}{3} = \frac{340}{3} = 113.33,$

plotted against the mid-year 2001.

Step 2 — second-half average (2004, 2005, 2006).

$\bar{y}_2 = \frac{110 + 125 + 135}{3} = \frac{370}{3} = 123.33,$

plotted against the mid-year 2005.

Step 3 — slope (annual trend increase). …