Undo / redo

If you already have serializing to files, you can reuse that serialization.
You should support Ctrl+Z for undo, and both Ctrl+Shift+Z and Ctrl+Y for redo - these are both very common shortcuts a user might try (Ctrl+Shift+Z is easier to type, Ctrl+Y is more of a Windows thing)
Here’s a simple model of undo history for reference:

undos = [];
redos = [];

function undo() {
    if (undos.length > 0) {
        redos.push(saveState());
        loadState(undos.pop());
    }
}

function redo() {
    if (redos.length > 0) {
        undos.push(saveState());
        loadState(redos.pop());
    }
}

function undoable() {
    undos.push(saveState());

    // DESTROY any redos (as is the common practice...)
    redos = [];
}

// then anywhere the document state is changed, dragging a node or anything,
// just call undoable() beforehand

and a more detailed example that might turn into an article someday: https://gist.github.com/1j01/bd2329547904b97abc52fd5e76b008d8

(please don’t take my over-explanation to mean i think you couldn’t do it all yourself, i just wanna make it easy/straightforward/approachable 🙂 (and i enjoy slowly refining my “simple undo example”))

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