Don't know what gentext to create for xref to "informalfigure" or "mediaobject"

What's wrong with the following docbook xml snippet:

    <para>Figure <xref linkend="fig.tour-basic.history"/> provides a
      graphical representation of the history of the <filename
	class="directory">hello</filename> repository, to make it a
      little easier to see which direction history is
      <quote>flowing</quote> in.  We'll be returning to this figure
      several times in this chapter and the chapter that
      follows.</para>

    <informalfigure>
      <mediaobject id="fig.tour-basic.history">
	<imageobject><imagedata fileref="images/tour-history.png"/></imageobject>
	<textobject><phrase>XXX add text</phrase></textobject>
	<caption><para>Graphical history of the <filename
	    class="directory">hello</filename> repository</para></caption>
      </mediaobject>
    </informalfigure>

How to xref 'imageobject' with 'caption' correctly?

Thanks,

---
Dongsheng Song