Dia shapes for chemistry

[Richard Muller <rpm wag caltech edu>]

I've just gone through the manuals and learned how to define new shapes 
and sheets for Dia. I'm really impressed with how easy it is, but had a 
few questions that I was hoping that someone could answer.

Here's an example of one of the structures I've defined:

<?xml version="1.0"?>

<shape xmlns="http://www.daa.com.au/~james/dia-shape-ns";
        xmlns:svg="http://www.w3.org/2000/svg";>
   <name>Chemistry - Benzene</name>
   <description>Basic six-membered ring</description>
   <icon>benzene.xpm</icon>
   <connections>
    <point x="1.0" y="0"/>
    <point x="0.5" y="0.866"/>
    <point x="-0.5" y="0.866"/>
    <point x="-1.0" y="0"/>
    <point x="-0.5" y="-0.866"/>
    <point x="0.5" y="-0.866"/>
   </connections>
   <svg:svg width="4.0" height="4.0">
     <svg:polygon points="1.0,0.0 0.5,0.866 -0.5,0.866 -1.0,0.0 
-0.5,-0.866 0.5,-0.866"/>
     <svg:line x1="0.8" y1="0" x2="0.4" y2="0.693"/>
     <svg:line x1="-0.4" y1="0.693" x2="-0.8" y2="0"/>
     <svg:line x1="-0.4" y1="-0.693" x2="0.4" y2="-0.693"/>
   </svg:svg>	
</shape>


Questions:
1. I would like to constrain the resizing so that if I change x, y is 
changed the same amount. Is there a way of doing this in SVG?

2. I would like to optionally label each vertex with a letter (e.g. "C" 
or "N"). Is there an easy way of doing this?

Thanks in advance for any help you can offer.

Rick
rpm@wag.caltech.edu