cytoscape save graph as image by button

You don't need server side code to save files from the browser anymore.

You can save files using the saveAs() API in JS. Here's a polyfill: https://github.com/eligrey/FileSaver.js/

If you want the graph data, it would just be

var jsonBlob = new Blob([ JSON.stringify( cy.json() ) ], { type: 'application/javascript;charset=utf-8' });

saveAs( jsonBlob, 'graph.json' );

Or for images

var b64key = 'base64,';
var b64 = cy.png().substring( content.indexOf(b64key) + b64key.length );
var imgBlob = base64ToBlob( b64, 'image/png' );

saveAs( imgBlob, 'graph.png' );

(Refer to other question re. base64toBlob())