mxGraphCodec.js

function CustomData(value) {
 this.value = value;
}

var codec = new mxObjectCodec(new CustomData());

codec.encode = function (enc, obj) {
 var node = enc.document.createElement('CustomData');
 mxUtils.setTextContent(node, JSON.stringify(obj));

 return node;
};

codec.decode = function (dec, node, into) {
 var obj = JSON.parse(mxUtils.getTextContent(node));
 obj.constructor = CustomData;

 return obj;
};

mxCodecRegistry.register(codec);