It seems graphoid keeps returning cache miss:
yurik@steppenwolf:~/wmf/graphoid/deploy$ curl -I https://graphoid.wikimedia.org/www.mediawiki.org/v1/png/Extension%3AGraph%2FDemo/1647673/3cbe2b968108670c001e230dca4682a9d03f8814.png
HTTP/1.1 200 OK Server: nginx/1.6.2 Date: Tue, 12 May 2015 16:26:12 GMT Content-Type: image/png Connection: keep-alive Access-Control-Allow-Origin: * Access-Control-Allow-Headers: Accept, X-Requested-With, Content-Type X-XSS-Protection: 1; mode=block X-Content-Type-Options: nosniff X-Frame-Options: SAMEORIGIN Content-Security-Policy: default-src 'self'; object-src 'none'; media-src *; img-src *; style-src *; frame-ancestors 'self' X-Content-Security-Policy: default-src 'self'; object-src 'none'; media-src *; img-src *; style-src *; frame-ancestors 'self' X-WebKit-CSP: default-src 'self'; object-src 'none'; media-src *; img-src *; style-src *; frame-ancestors 'self' Cache-Control: public, s-maxage=30, max-age=30 Accept-Ranges: bytes X-Varnish: 1283394768 Age: 0 Via: 1.1 varnish X-Cache: cp1045 frontend miss (0)
P.S. I have updated Graphoid to set Cache-Control to 30 seconds on both 200 & 400