Beginner help with multiple queries in a pie or gauge

I am trying to graph total / used / available disk space on my isilon. The data is coming from Zabbix.

I have:

ifsFreeBytes
ifsTotalBytes
ifsUsedBytes

How do I combine these into one guage or Pie graph? I have the pigraph installed, but it doesn’t want to show all 3 correctly. Gauge just shows 3 different gauges.

I want to show the Total, with the free and used taken out of it. Percentages would be nice too.

Sorry if this is really basic.