Proportional and linear relationships
Proportional
directly proportional (y = mx)
For example using Ohm's Law, potential differece is proportional to the current through an Ohmic resistor at a fixed temperature.
Intercept should be zero if you graph: y vs x
inversely proportional (y = m/x)
For example when investigating drag, the velocity squared is inversely proportional to the surface area.
Do not graph the inverse relationship y vs x, graph y vs 1/x to get a directly proportional relationship.
Intercept should be zero if you graph: y vs 1/x
Linear
(y = mx + c)
For example the voltage generated across a solar panel with changing temperature is linear, in the form: V(a) = k ΔT + V(r)