EXtjs : Checkbox group model in combobox

I am new to extjs,

I have a requiremnt in which i have to create a dynamic populated multicolumn checkbox model in the combobox drop down. I am using extjs 4.2

Can u please let me now how to do this..

||||| Drop down |||||||||||| | chkbox1 chkbox8 chkbo15 | | chkbox2 chkbox9 chkbo16 | | chkbox3 chkbo10 chkbo17 | | chkbox4 chkbo11 chkbo18 | | chkbox5 chkbo12 chkbo19 | | chkbox6 chkbo13 chkbo19 | | chkbox7 chkbo14 chkbo20 | ||||||||||||||||||||||||||||||

And this check box values needs to be populated from the ajax request how to proceed on this..its urgent