If you didn't cancel in time, then you bought another year of service. Linode don't have to give you a refund, but they usually do, because they are good guys. I'll bet that giving out refunds just isn't at the top of their to do list. Check the Linode ToS before you try to get your card issuer to do a recharge.
Linode ToS, para 5 wrote:
Refund and Disputes: All payments to Linode.com are nonrefundable. This includes the one time setup fee and subsequent charges regardless of usage. All overcharges or billing disputes must be reported within 60 days of the time the dispute occurred. If you dispute a charge to your credit card issuer that, in Linode.com's sole discretion is a valid charge under the provisions of the TOS and /or AUP, you agree to pay Linode.com an "Administrative Fee" of not less than $50 and not more than $150.