complicated xslt question

Hi!
 I'm trying to write XSL for a statistics page. (see XML below)
 Each site has four values to analyse: v1, v2, v3 and v4.

 The result of the transformation should be a set of tables for each
 month and one summarizing table.
 First I need  to calculate sum of each value. For example for March it
 is v1_sum=100+100.
 Next I need to find maximum value among summarized values of v1. This
 is v1_max=max(200,330)=330
 Here is an example:


 March
     | v1  v2  v3  v4
 -----------------------
 site1 | 100 200 300 400
 site2 | 100 200 300 400
 -----------------------
 sum   | 200 400 600 800


 April
     | v1  v2  v3  v4
 -----------------------
 site1 | 110 210 310 410
 site2 | 110 210 310 410
 site2 | 110 210 310 410
 -----------------------
 sum   | 330 630 930 1230


 SUMMARY
 -----------------------
 v1_max | 330
 v2_max | 630
 v3_max | 930
 v4_max | 1230



 I know how to find maximum among v1 values.
 <xsl:variable name="v1_max">
       <xsl:for-each select="month/@v1">
               <xsl:sort data-type="number" order="descending"/>
               <xsl:if test="position()=1"><xsl:value-of select="."/></xsl:if>
       </xsl:for-each>
 </xsl:variable>

 But in the case described I need to find the max in the set of values
 that has to be built first. Does anyone have any suggestions how to do
 that?
 Thanks!

 XML
 *****************
 <?xml version="1.0" encoding="UTF-8"?>
 <stats>
       <month name="March, 2008">
               <site id="1" v1="100" v2="200" v3="300" v4="400"/>
               <site id="2" v1="100" v2="200" v3="300" v4="400"/>
       </month>
       <month name="April, 2008">
               <site id="1" v1="110" v2="210" v3="310" v4="410"/>
               <site id="2" v1="110" v2="210" v3="310" v4="410"/>
               <site id="3" v1="110" v2="210" v3="310" v4="410"/>
       </month>
 </stats>