You could specify the column footers by use of listfoot and listfooter as follows. Notice that the order of listhead and listfoot doesn't matter. Each time a listhead instance is added to a list box, it must be the first child, and a listfoot instance the last child.

<listbox width="200px">
    <listhead>    
        <listheader label="Population"/>        
        <listheader align="right" label="%"/>        
    </listhead>    
    <listitem id="a" value="A">    
        <listcell label="A. Graduate"/>        
        <listcell label="20%"/>        
    </listitem>    
    <listitem id="b" value="B">    
        <listcell label="B. College"/>        
        <listcell label="23%"/>        
    </listitem>    
    <listitem id="c" value="C">    
        <listcell label="C. High School"/>        
        <listcell label="40%"/>        
    </listitem>    
    <listitem id="d" value="D">    
        <listcell label="D. Others"/>        
        <listcell label="17%"/>        
    </listitem>    
    <listfoot>    
        <listfooter label="More or less"/>        
        <listfooter label="100%"/>        
    </listfoot>    
</listbox>