Graphing issue with data that only changes once every 24 hours

[Phil Meech]

Hi All,

I've been working on a VS backup solution which I have implemented
using proftpd with mySQL, ftpmail (perl script) and quotas on Debian.
Essentially whenever a client uploads data, ftpmail reads the transfer
log from a fifo, and I've edited the perl script to send status
messages through to Xymon.  The solution is provided as only being
used for backups and the client software sends in its own logs to our
Xymon server.  This is a differential backup and the client software
handles the deletion of aged backup files.

All is working robustly on this front and in Xymon I collect the
following data for graphing:

 usage : 83.09
 allowance : 100.00

These figures are currently sent in MB (I need to write a percentile
calculation), but only once every 24 hours.  This all falls over when
it comes to trying to graph the data in Xymon as it is not a constant
feed of data every 300 seconds like other tests.  From the rrdtool
documentation it seems like the data I am presenting is unsuitable for
graphing in the normal method.  I've tried just about every data type
and I only get a line drawn when data is received (else the graph
drops out on every step without data).

I realise I could run a client test on the VS that runs du on each
chrooted FTP folder every 5 minutes, but this seems like an
unnecessary load when the data changes with the 24 hour frequency and
it scales horribly.  Also throwing out ~100 status messages for ~100
different clients every few minutes to one server (say if I was to
only run the test every 24 hours and repeat the data sent every 5
minutes) may be a little unreliable.  Possibly changing the xymon
client on the VS to a server and running everything locally would be
the way to go; but still load wise this isn't great when compared to
the methodology of only sending data on upload - though functional I
guess.

Is there a way of providing a custom step in rrdtool for a single test
from Xymon?  Has anyone got any other ideas of how I might graph this?
 Am I missing something obvious?

Cheers,
Phil