add serverGraphDuration config option

lib/time.js

@@ -1,7 +1,6 @@
-const SERVER_GRAPH_DATA_MAX_LENGTH = require('./servers').SERVER_GRAPH_DATA_MAX_LENGTH
-
 class TimeTracker {
-  constructor () {
+  constructor (app) {
+    this._app = app
     this._points = []
   }
 
@@ -10,7 +9,7 @@ class TimeTracker {
 
     this._points.push(timestamp)
 
-    if (this._points.length > SERVER_GRAPH_DATA_MAX_LENGTH) {
+    if (this._points.length > this._app.pingController.getMaxServerGraphDataLength()) {
       this._points.shift()
     }