title: how to do a scatter plot with empty circles in Vtk
body: <p>I draw a  2d scatter plot with vtk, however points are full circles.
  I would like to know how to do a scatter plot with empty circles please ?
  I use this example for draw my scatterplot <a href="http://www.itk.org/Wiki/VTK/Examples/Cxx/Plotting/ScatterPlot" rel="nofollow">http://www.itk.org/Wiki/VTK/Examples/Cxx/Plotting/ScatterPlot</a></p>