Support negative graph values.

Currently graph Y-Axes were hardcoded to start at 0. Choose the Y-scale
automatically based on the graph data instead.

web/static/js/graph.js

@@ -403,7 +403,8 @@ Prometheus.Graph.prototype.showGraph = function() {
     width: Math.max(self.graph.innerWidth(), 200),
     renderer: (self.stacked.is(":checked") ? "stack" : "line"),
     interpolation: "linear",
-    series: self.data
+    series: self.data,
+    min: "auto",
   });
 
   var xAxis = new Rickshaw.Graph.Axis.Time({ graph: self.rickshawGraph });