To get an HP48 ploting the frequerncy response of a 
Z transform, first define Z as 'EXP(X*(0,-1)).

i.e.

> 'EXP(X*(0,-1))'
> 'Z'
> STO

Now define a function to convert a Z transform
to ABS and then to DBs 

> << ABS LOG10 10 * >>
> '->ABS'
> STO

Now place in a test function say the 7/8ths LAG FILTER

> ' 0.125/(1-0.875*Z)'
> ->ABS

Now store in 'EQ'

Now get the plot menu.
Set yrange to -40 to + 40
Set xrange to 0 to 3.142

PLOT func

The Z transform magnitude response will now be displayed in the 
HP48 graph