What is the constant of proportionality represented in the graph?

This constant of proportionality is equal to the slope of the line

If we asume each rectangle side is equal to a unit, then

Point 1 (0,3)
Point 2 (5,-1)

m = (y2 - y1) / (x2-x1)

m = (-1 - 3) / (5-0)

m = (-4) / (5) = -4/5 = k