MatterViz (the `matterviz` npm package) is a web‑based visualization toolkit for materials‑science data, providing ready‑to‑use Svelte components that render 3D crystal structures, molecules, molecular‑dynamics trajectories, periodic‑table heatmaps, phase diagrams, convex hulls, band structures, density‑of‑states, Brillouin zones and related plots. It targets researchers, educators and developers who work with computational materials data and want interactive visualisations directly in browsers or within VS Code via the MatterViz extension.