hAxes = axes('NextPlot', 'add', 'XLim', [0, 20], 'YLim', [-1, 10], 'Position', [0 0 0.5 0.5]);
h2Axes = axes('NextPlot', 'add', 'XLim', [0, 20], 'YLim', [-1, 10], 'Position', [0.5 0.5 0.5 0.5]);
W = 4;
Y = 0;
H = 8;
for i = 1 : 3
X = i.* W;
R = rectangle('Parent', hAxes);
% Or:
% R.Parent = hAxes;
R.Position = [X Y W H];
G(i).r = R;
end