add serverGraphDuration config option

lib/ping.js

@@ -151,6 +151,14 @@ class PingController {
       }, version.protocolId)
     }
   }
+
+  getMaxServerGraphDataLength () {
+    return Math.ceil(config.serverGraphDuration / config.rates.pingAll)
+  }
+
+  getMaxGraphDataLength () {
+    return Math.ceil(config.graphDuration / config.rates.pingAll)
+  }
 }
 
 module.exports = PingController