A bit of a kludge, but until @alt is available, change the xsl to:
<figcaption>Figure <xsl:value-of select="substring(@src,string-length($title)+2,2)"/></figcaption>
This takes the name of the image file (myfunction-xx.png) and pulls out the xx, so the first figure will be "Figure 01". Yes, I could probably figure out how to suppress the leading zero.
What would be a nice feature would be the ability to define an XML variable in the script, so you could cross reference to the figure, something like:
setXMLtag('fignum', 1);
setXMLtag('caption', 'Money as a function of time');
%%
% As can be seen in Figure @fignum, time is money
plot(time,money)