gain_combo=TTK.Combobox(window,width=4)
gain_combo['values']=(1,2,4,8,16)
gain_combo.current(0)
gain_combo.grid(column=1,row=3)

# add checkboxes
plot_dOD_state=tk.BooleanVar()
plot_dOD_state.set(False)
plot_dOD_chk=tk.Checkbutton(window,text='Plot delta OD',var=plot_dOD_state)
plot_dOD_chk.grid(column=1,row=5)