diff --git a/src/routes/graphs.js b/src/routes/graphs.js
index 0f1136f4e..084b591be 100644
--- a/src/routes/graphs.js
+++ b/src/routes/graphs.js
@@ -8,6 +8,7 @@ var middleware = require('../middleware/index.js'),
     url = require('url');
 
 // for testing locally: curl 'http://127.0.0.1:8417/graphite-web/render?format=json&from=-1min&target=absolute(collectd.localhost.du-docker.capacity-usage)'
+// the datapoint is (value, timestamp) https://buildmedia.readthedocs.org/media/pdf/graphite/0.9.16/graphite.pdf
 const graphiteProxy = middleware.proxy(url.parse('http://127.0.0.1:8417'));
 
 function getGraphs(req, res, next) {