Visual diagrams are ubiquitous elements in science communication and science teaching. We can find them in texts from any area of knowledge. However, more studies are required to understand the mechanisms that make them useful tools for sharing information. This research, from an information design perspective, analyzes visual diagrams by identifying the morphological features that define them and proposing an original classification. This is a qualitative exploratory and descriptive study, based on non-probabilistic sample techniques, and through a dialectic process some proposals have been integrated from specialised literature, historical records research, practical contemporary applications analysis, information design theory, science communication and the practical experience of the authors. As a result, a diagram classification is proposed which comprises four morphological structures. Visual diagrams have been identified as useful tools to explain scientific topics, because their structure is practical for adding new characteristics in order to be adapted to the requirements of communicating knowledge visually. The insertion of information design leverages and promotes visual and information literacy capabilities both in the construction of visual diagrams.