Any chance you could add actual pen-like drawing? That would make it much more versatile, and make it fit its description better ("Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them."). Bonus points for using PointerEvents to change the line width with the pen's pressure.
Saving the diagram automatically and restoring when the site is revisited is a nice touch.
Saving the diagram automatically and restoring when the site is revisited is a nice touch.