You have to set some sort of symbol for the graphics being added to the map. What if you change defaultSymbol to be transparent? dojo colors can be constructed with an alpha paramter ranging from 0 = transparent to 1.0.
new dojo.Color([r,g,b,a])
defaultSymbol = new esri.symbol.SimpleMarkerSymbol().setColor(new dojo.Color([0,0,255,0]));