Because the constant of proportionality only exists for linear relationships, you can use the graph of a proportional relationship to figure out what your constant of proportionality is.
To find your constant of proportionality from your graph, you are basically finding the slope of your line. You want to locate two points with easy coordinates, then you count the number of squares you need to go up followed by the number of squares needed to go to the right to get from your first point to the second point, your first point being to the left of your second point.