This is the expected behavior - as explained in the Understanding Report Sections
help article, the detail section is printed once for every row in the data source and it is the section where you place the report items that make up the main body of the report. Thus all items placed in the report detail section would be repeater as many times as the data rows in your database (or xml file in this case).
If you want to display the chart according to certain records only, you should consider using grouping and place the chart in a group header/footer section where it would be displayed once for each group and you can filter it according to the group criteria. Other approaches are possible as well, depending on your exact scenario which we are not aware of.
the Telerik team