Panel a: Raw RVs less the systemic velocity of K2-18. Panel b: RV contribution from stellar activity; raw RVs corrected by the MAP Keplerian orbital solutions for each detected planet. Panel c: RV contribution from K2-18b; raw RVs corrected for activity and K2-18c. Panel d: RV contribution from K2-18c; raw RVs corrected for activity and K2-18b. Panel e: the RV residuals. The solid curves in panels b, c, and d depict the mean GP activity model, and the MAP Keplerian models for K2-18b and c) respectively. The surrounding shaded region in panel b is the 68% confidence interval on the mean GP model. All RV units are in m s^-1.