Parse custom step parameter correctly. (#2928)

Backport of 6a767b736b.
Refs #2827, #2861.

web/ui/static/js/graph.js

@@ -364,7 +364,7 @@ Prometheus.Graph.prototype.submitQuery = function() {
 
   var startTime = new Date().getTime();
   var rangeSeconds = self.parseDuration(self.rangeInput.val());
-  var resolution = self.queryForm.find("input[name=step_input]").val() || Math.max(Math.floor(rangeSeconds / 250), 1);
+  var resolution = parseInt(self.queryForm.find("input[name=step_input]").val()) || Math.max(Math.floor(rangeSeconds / 250), 1);
   var endDate = self.getEndDate() / 1000;
 
   if (self.queryXhr) {