> but I'm not sure how to append to a variable.
You can't change the value of a variable once it's bound, but you don't
need any variables here.

<svg:path>
 <xsl:attribute  name="d">
  xsl:for-each select="value">
  <xsl:choose>
  <xsl:when test"position()=1">M</xsl:when>
  <xsl:otherwise>L</xsl:otherewise>
  </xsl:choose>
  <xsl:value-of select="@seconds"/>
  <xsl:text> </xsl:text>
  <xsl:value-of select="@LATENCY"/>
 </xsl:for-each>
</xsl:attribute>
</svg:path>

David