Multiwavelength polarization versus time of Mrk 421. Each row presents the polarization degree (circle) and polarization angle (triangle), with 1σ errors measured by radio (f < 50 GHz in red, 50 < f < 100 GHz in yellow, 100 < f < 200 GHz in green, and f < 200 GHz in blue), infrared (green), optical (red), and X-ray (orange) facilities, from top to bottom. The gray shaded areas denote the durations of the four IXPE observations.