Using coordinates O(0,0), E(0, 2b), and F(2a, 0) rather than O(0, 0), E(0, b), and F(a, 0) has the advantage of allowing the graph to be stretched or shrunk uniformly in both the x- and y-directions by a factor of 2. This can be useful if the scale of the graph needs to be adjusted, or if the graph needs to be compared to another graph with a different scale. It can also make the graph easier to read if the values of the variables being plotted are very large or very small.
By using coordinates that are multiples of the original coordinates, the shape of the graph is preserved, but the scale is changed. This can be useful if the purpose of the graph is to compare the relationships between different variables, rather than to provide an accurate representation of the actual values of the variables.