By default, Xymon’s trends show metrics for the last 48 hours. Should you want to “only” get the last 24 hours on the main “trends” page, you have to edit the xymon/server/etc/xymonserver.cfg and edit or add the TRENDSECONDS variable. 86400 will render only the last 24 hours.
Note that TRENDSECONDS is optimized for the default RRDWIDTH and INTERVAL values. Setting random or too small TRENDSECONDS may lead to ugly graphics rendering.