Three friends are buying seashells at the gift shop on the beach. Melanie buys 2 seashells for​ $0.80. Rosi buys 5 seashells for​ $2.00. Carlos buys 4 seashells for​ $1.60. Use a graph to determine whether the number of seashells and the cost have a proportional relationship. If​ so, what is the constant of proportionality and what does it​ mean?