Fix zoom stops working when DOT source is cleared

Fixes #135.

The zoom functionality stores the original "graph0" <g> element and
applies zoom transforms to it, but that elements was removed when the
DOT source was cleared.

Instead of removing the whole svg, now only the graph elements below
the "graph0" <g> element are removed.

cypress/integration/browser_save_and_open.spec.js

@@ -609,7 +609,8 @@ describe('Browser save and open', function() {
 
     cy.savedGraphs().should('have.length', 0);
 
-    cy.canvasSvg().should('not.exist');
+    cy.nodes().should('have.length', 0);
+    cy.edges().should('have.length', 0);
   })
 
   it('The graph is renamed in browser local storage through the menu item Rename', function() {
@@ -968,7 +969,8 @@ describe('Browser save and open', function() {
 
     // FIXME: Temporary workaround to ensure that the new SVG will be stored in localStorage
     cy.clearDotSource();
-    cy.canvasGraph().should('not.exist');
+    cy.nodes().should('have.length', 0);
+    cy.edges().should('have.length', 0);
 
     cy.clearAndRenderDotSource('digraph {Bob}');
 
@@ -1101,7 +1103,8 @@ describe('Browser save and open', function() {
 
     // FIXME: Temporary workaround to ensure that the new SVG will be stored in localStorage
     cy.clearDotSource();
-    cy.canvasGraph().should('not.exist');
+    cy.nodes().should('have.length', 0);
+    cy.edges().should('have.length', 0);
 
     cy.clearAndRenderDotSource('digraph {Alice}');
 

src/Graph.js

@@ -39,6 +39,7 @@ class Graph extends React.Component {
       busy: false,
     };
     this.svg = d3_select(null);
+    this.graph0 = d3_select(null);
     this.createGraph = this.createGraph.bind(this)
     this.renderGraph = this.renderGraph.bind(this)
     this.isDrawingEdge = false;
@@ -108,7 +109,7 @@ class Graph extends React.Component {
     let fit = this.props.fit;
     let engine = this.props.engine;
     if (this.props.dotSrc.length === 0) {
-      this.svg.remove();
+      this.graph0.selectAll('*').remove();
       this.svg = d3_select(null);
       this.props.onError(null);
       this.renderGraphReady = false;