After you have the dots plotted, ther are three steps to find the trend line on a scatter plot:
1) Draw a vertical line that separates the full set of points into two sets, each with the same number of dots.
2) Mark with a special sign (e.g + or * or x) the middle value in each of the two sets obtained in the previous step.
3) Join the two marks mentioned in the point 2) with a straight line, and extend it to the edges of the graph. This line is the trend line, and this is the last step.
The last step is drawing the straight line that joins the middle points of each half of the scatter plot..
