Add an option to disable chart legend.

A user can disable it if he has tons of series.
Now that we explicitly enable it, it's also visible for a single series.

rd_ui/app/scripts/visualizations/chart.js

@@ -8,6 +8,7 @@
     var defaultOptions = {
       globalSeriesType: 'column',
       sortX: true,
+      legend: {enabled: true},
       yAxis: [{type: 'linear'}, {type: 'linear', opposite: true}],
       xAxis: {type: 'datetime', labels: {enabled: true}},
       series: {stacking: null},
@@ -180,6 +181,10 @@
           }
         });
 
+        if (!_.has(scope.options, 'legend')) {
+          scope.options.legend = {enabled: true};
+        }
+
         if (scope.columnNames)
           _.each(scope.options.columnMapping, function(value, key) {
             if (scope.columnNames.length > 0 && !_.contains(scope.columnNames, key))