Grouping of BarChart and BarChart labels

Thanks to m_goldberg remarks here, the following BarChart was obtained:

data14GAOF = {{33/36, 3/36}, {11/36, 25/36}, {27/36, 1/36,      8/36}, {14/36, 22/36}, {1/36, 13/36, 1/36, 1/36, 2/36, 2/36,      10/36, 4/36, 1/36, 1/36}, {8/36, 8/36, 16/36, 4/36}}; labels14GAOF = {{"node 4", "node 9"}, {"node 4", "node 9"}, {"node 4",      "node 7", "node 9"}, {"node 4", "node 9"}, {"node 3", "node 4",      "node 5", "node 6", "node 7", "node 8", "node 9", "node 10",      "node 11", "node 13"}, {"node 4", "node 5", "node 9", "node 11"}}; grouplabels14GAOF =    Style[#, Black, Bold, FontSize -> 18] & /@ {"OF1(N-1)", "OF1",      "OF2(N-1)", "OF2", "OF3(N-1)", "OF3"}; labeleddata14GAOF =    MapThread[    Labeled[#1, #2, Axis] &, {SortBy[-First[#] &] /@       Apply[Labeled[#1, Style[#2, FontSize -> 16, White], Center] &,        Transpose /@ Transpose[{data14GAOF, labels14GAOF}], {2}],      grouplabels14GAOF}]; BarChart[labeleddata14GAOF,   ChartStyle -> {GrayLevel[0.1], GrayLevel[0.2], GrayLevel[0.3],     GrayLevel[0.4], GrayLevel[0.5], GrayLevel[0.6], GrayLevel[0.65],     GrayLevel[0.7], GrayLevel[0.75], GrayLevel[0.8]},   ChartLayout -> "Stacked", AxesStyle -> Directive[Black, 24],   BarSpacing -> {Automatic, .3}, ImageSize -> 1200] 

enter image description here

It is correct but it would be better to sowehow group the columns. In other words to separate each two columns OF1(N-1) and OF1, OF2(N-1) and OF2, OF3(N-1) and OF3 from each other a bit because they represent different functions. The idea is to get something like that:

enter image description here

How does it can be realised?