iReport How to group data from xml

Question

Hello all i don't understand something very fundamental in Ireport and its driving me crazy i have simple xml data source:

<a>
    <a1>
         <a2>
         </a2>
    </a1>
     <a1>
         <a2>
         </a2>
    </a1>
</a>
<b>
    <b1>
         <b2>
         </b2>
    </b1>
     <b1> 
         <b2>
         </b2>
    </b1>
</b>

now in my report i like to order the nodes one after the other and after this the nodes . but when i try place the nodes in the designer fields like this $F{a1} $F{b1} what im getting in the preview is mix of both fields like this :

<a2>
<b2>
<a2>
<b2>

and i expect to get

<a2>
<a2>
<b2>
<b2>

what im doing wrong here ?