People with visual impairments or blindness (VIB) encounter diffculties in exploring graphical representations that are widely used for the study of STEM subjects. In particular, graphs are used to represent many different scientifc notations: fowcharts, automata, cognitive maps, and more. Among these, structural chemical formulae are characterized by a complex, often 3-dimensional structure, which makes them hard to access and author with traditional assistive tools.We propose µGraph, a multimodal system that combines haptic and speech feedback to enable people with VIB to explore and edit structural chemical formulae. Two main contributions are presented: (i) a novel, non-visual interaction paradigm for exploring graphs and its implementation in the µGraph system, and (ii) an extensive evaluation of the proposed system with 10 participants with VIB showing that µGraph is thoroughly accessible and that the haptic feedback enhances understanding of the geometric properties of a graph.