Migrate to a D3-based graph renderer

index.html

@@ -21,8 +21,7 @@
         <div id="input-buttons"></div>
         <button id="clear-button">Clear</button>
       </div>
-      <pre id="pen"></pre>
-      <pre id="state-graph"></pre>
+      <div id="state-graph"></div>
     </div>
     <script type="module" src="src/main.js"></script>
   </body>