Graph panel: display only bars in histogram mode, issue #5812.

public/app/plugins/panel/graph/graph.js

@@ -247,6 +247,9 @@ function (angular, $, moment, _, kbn, GraphTooltip) {
             if (data.length) {
               options.series.bars.barWidth = 0.7;
               options.series.bars.align = 'center';
+              options.series.bars.show = true;
+              options.series.points.show = false;
+              options.series.lines.show = false;
             }
 
             addXAxis(options);