Disabled checkbox in dojo datagrid

I am new with dojo and I am trying to create a datagrid with disabled checkboxes in it.

var grid = new DataGrid({
    id: 'target_id',
    store: new ObjectStore({
      objectStore: myData
    }),
    query: {parent: 'root'},
    structure: layout,
    rowSelector: '0px'
  });

layout[0] = {name: ' ', field: 'selected', width: '20px', editable: false, cellType: dojox.grid.cells.Bool};

But instead of disabled checkboxes , the only thing I can see is "true" or "false". True if the disabled checkbox should be checked and false if not

And this is how it looks like if I switch editable to true

How can I make the checkboxes visible (instead of "true" and "false", but disabled?)