Is Viscosity = Slope?

studying fluid mechanics, I see that viscosity relates shear stress to shear rate, like a slope. However, on graphs I see that they draw a triangle, and where I would expect to see $Delta tau $, I see $mu_{ap}$ instead. Why is the slope being "drawn" along the y-axis, and not represented as the usual math slopes I have come across: $$frac{dtau} {d dot{gamma}}$$.