Use proper name of column in tooltip instead of first one

- determine name of column (Header column) from chart column.
Name is taken from MiqReport#headers by index.
This index is determined from MiqReport#col_order

app/models/miq_report.rb

@@ -224,4 +224,16 @@ def remove_loading_relations_for_virtual_custom_attributes
     vc_attributes = CustomAttributeMixin.select_virtual_custom_attributes(cols).present?
     include.delete(:custom_attributes) if vc_attributes.present? && include && include[:custom_attributes].blank?
   end
+
+  # determine name column from headers for x-axis in chart
+  def chart_header_column
+    if graph[:column].blank?
+      _log.error("The column for the chart's x-axis must be defined in the report")
+      return
+    end
+
+    chart_column = MiqExpression::Field.parse(graph[:column]).column
+    column_index = col_order.index { |col| col.include?(chart_column) }
+    headers[column_index]
+  end
 end

lib/report_formatter/chart_common.rb

@@ -407,7 +407,7 @@ def build_numeric_chart_simple
       # Pie charts put categories in legend, else in axis labels
       add_axis_category_text(categories)
 
-      add_series(mri.headers[0], series)
+      add_series(mri.chart_header_column, series)
     end
 
     def build_numeric_chart_grouped
@@ -438,7 +438,7 @@ def build_numeric_chart_grouped
       # Pie charts put categories in legend, else in axis labels
       add_axis_category_text(categories)
 
-      add_series(mri.headers[0], series)
+      add_series(mri.chart_header_column, series)
     end
 
     def build_numeric_chart_grouped_2dim
@@ -537,7 +537,7 @@ def build_reporting_chart_other
 
       # Pie charts put categories in legend, else in axis labels
       add_axis_category_text(categories)
-      add_series(mri.headers[0], series)
+      add_series(mri.chart_header_column, series)
     end
 
     # C&U performance charts (Cluster, Host, VM based)