This behavior is expected - line series draw lines across their items. If you want to draw just a line I can suggest the following:
- Use PlotBands - http://demos.telerik.com/aspnet-ajax/htmlchart/examples/functionality/plot-bands/defaultcs.aspx
- Use Visual Templates - http://demos.telerik.com/aspnet-ajax/htmlchart/examples/functionality/visual-templates/defaultcs.aspx
- Directly utilize the Kendo Drawing API, to draw the line - http://www.telerik.com/forums/custom-drawing
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