The most important thing at the moment of determining proportions between graphs in a coordinate plane, is that the scale of each axis in the coordinate plane are the same. That is, the units that you use for horizontal axis must be equal to units that you use for vertical axis.
Then, the reached proportions comes from the fact of making quotient between specific sides of segment of a graph. For example, you have two graphs and you want to know about thier proportions. You measure the length, of a segment of one graph, mean AB, and the segment of another graph, mean CD, and a convenient proportion can be obtained from:
p = AB/CD