Adhering to anti-hypertensive medications contributes to control of blood pressure and improved health outcomes. However, adherence rates among older adults are low. Patient monitoring of medication taking helps increase adherence and technology has great potential to support self-monitoring, in part by providing visual feedback about medication taking performance. However, little attention has been paid to designing feedback visualizations in medication-monitoring technology for older adults. In this research, we identified guidelines for designing understandable and effective adherence visualizations for older adults from existing theories and literature. With the guidelines in mind, we designed, refined, and evaluated visualizations that provided adherence feedback for a smartphone application with 17 older participants. Based on theory and evidence, we identified design guidelines for feedback visualizations. These guidelines can support design of useful feedback visualizations that may improve medication adherence among older adults.