It seems graphoid keeps returning cache miss:
```lang=bash
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