Default to auto scaling for grid min setting

fixes #226

src/app/dashboards/default.json

@@ -48,7 +48,7 @@
           "y_formats": ["short", "short"],
           "grid": {
             "max": null,
-            "min": 0
+            "min": null
           },
           "resolution": 100,
           "lines": true,
@@ -150,4 +150,4 @@
     "hide": false
   },
   "refresh": false
-}
+}

src/app/panels/graphite/module.js

@@ -94,7 +94,7 @@ function (angular, app, $, _, kbn, moment, timeSeries) {
        */
       grid          : {
         max: null,
-        min: 0,
+        min: null,
         threshold1: null,
         threshold2: null,
         threshold1Color: 'rgba(216, 200, 27, 0.27)',