[Therion] Showing centreline for whole cave

Hi, We're in the process of trying to draw up an extended elevation of a complicated part of one cave. It would be very useful to see the centreline plotted for the whole cave. I've added the metapost code from http://therion.speleo.sk/wiki/doku.php?id=metapost and it plots the whole line rather than just the ends (though it is only showing for half of the cave). I've tried adding thee names of surveys which are not in the scraps to the map, but no centreline is shown. Does anyone have any advise? Is there a way to just get all the centreline plotted? Thanks, John