Problem with graphviz on Unix

Question

When I'm on Unix, my graphs with graphviz are less nice. The tail or the head of the edge is not always linked to the boundary of the port. You can see the result here : http://www.freeimagehosting.net/image.php?59d8eb4b30.jpg

<xsl:value-of select="$source"/>:<xsl:value-of select="$prot"/> 
    -&gt; <xsl:value-of select="$destination"/>:<xsl:value-of select="$prot"/>[color="black",label="<xsl:value-of select="$prot"/>",fontcolor="black",tailclip=true,headclip=true];