Blazars are active galactic nuclei whose relativistic jets ejected from the central black hole are pointing toward the Earth. Astronomers have attempted to classify blazars, but analyzing the time-dependent multivariate datasets with conventional visualization methods, such as scatter plot matrices, is difficult. This paper presents TimeTubes, a new visualization scheme that allows astronomers to analyze dynamic changes in and feature causality among the multiple time-varying variables. We target six representative time-varying variables from the originals, including two polarization-related parameters and their corresponding errors, intensity, and color. The four polarization parameters with a common time stamp are transformed to an ellipse, and a series of such ellipses are aligned in parallel along the time line to form a volumetric tube in 3D space. The resulting tube is then colorized by the observed intensities and colors of the blazar. We designed a designated interface with nine functions to control the view of the tube interactively. The usability of TimeTubes is discussed with feedback from astronomers.