Multiple plots in one display

Say I have a field of mathtype (time -> (a,b))
I can successfully plot (time vs a) or (time vs b) on a Display but
I want to know if it possible to plot time vs a & b. 
i.e a chart with two lines.
Can anyone help?

Cameron Shepherdley <C.Shepherdley@xxxxxxxxxx>
AutoTAF Project Developer 
Bureau of Meteorology, Melbourne, VIC 3001, Australia
Telephone: +613 9669 4652