From left to right: observed surface density perturbation in the millimeter dust continuum of the IM Lup disk normalized to the radial symmetric surface density (a), the perturbation in radial velocity normalized to the Keplerian velocity, derived from the surface density perturbation using Eq. (14) (b), and the azimuthal velocity perturbation normalized to the Keplerian velocity derived from δv_r,calc using Eq. (9) (c). The dashed lines represent a fit by eye through the maximum of the spiral in the surface density, which serves as a guide for the eye.