Xavier claims that the number of text messages he sends is proportional to the amount he owes on his monthly phone bill. He has listed the number of text messages sent and the amount he owed for a few months: January: 300 text messages sent: $15.00 owed February: 314 text messages sent; $15.70 owed March: 486 text messages sent; $24.30 owed April: 205 text messages sent; $10.25 owed Which of the following methods would help Xavier prove his claim?