Thanks, @sebastian. It wokred. For future reference, this is what worked:
function add_rectangle(src, event)
a = get(src,'Tag')
if evalin('base', 'exist(''tag_list'',''var'')')
tag_list= evalin('base','tag_list');
else
tag_list= {};
end
class(tag_list)
tag_list{end+1} = {a}; % add the point
assignin('base','tag_list',tag_list); % save to base
end