#478 API crashes on 2nd render() call

- add e2e test for (re)rendering by api

cypress/integration/other/rerender.spec.js

@@ -0,0 +1,16 @@
+/* eslint-env jest */
+describe('Rerendering', () => {
+
+    it('should be able to render and rerender a graph via API', () => {
+      const url = 'http://localhost:9000/rerender.html';
+      cy.viewport(1440, 1024);
+      cy.visit(url);
+      cy.get('#graph #A').should('have.text', 'XMas');
+
+      cy.get('body')
+        .find('#rerender')
+        .click({ force: true });
+
+      cy.get('#graph #A').should('have.text', 'Saturday');
+    });
+});