To complete the equation of the graph with its constant of proportionality, you would need to know the specific values of weight in ounces (x) and cost (y) illustrated in the image you referenced.
Once you have those values, you can determine the constant of proportionality (k) by using the formula \( k = \frac{y}{x} \). The equation would then be written as:
\[ y = kx \]
Without the specific numbers from the image, I can't provide the exact constant. If you supply me with the values from the graph, I can help you compute it!