Background: To support collaborative drawing, it is essential to investigate how students make collaborative drawings and how these contribute to elaborating their ideas. This study examines how 5th and 6th grade students' group drawings contributed to increased levels of explanations of their drawings about sound transmission. Methods: We analyzed two cases of group drawing processes, that showed a large difference in the explanatory levels in their drawings, to find discourse patterns and visualized these patterns through discourse maps in relation to the progressions of drawing. Findings: In the first case, the students successfully co-constructed sound transmission drawings following Demand-Give-Acknowledge patterns. The students continuously questioned how to visualize particles' vibration, used multimodal resources to generate alternative drawings, and determined most scientific drawings. In the second case, the students did not reach consensus on how to visualize particles' vibrations, following repetitive patterns of Give-Refute. While the teacher intervened and mediated student's conflicting ideas, the students did not generate any alternative ideas. Contribution: This study illustrates in close detail how the process of multimodal transactive discussion contributed to conceptual understanding during collaborative drawings. The discourse map may be instrumental to analyze students' collaboration systematically and devise pedagogical approaches.