family waveform expressions

Greetings all,

I'm running IC6.1.500.6

I'm running into the problem where if I create a family waveform using the standard family waveform functions (famCreateFamily, famAddValue), it is creating plots that are not differentiated between each other in viva.

e.g.

a = family waveform created with famCreateFamily
b = family waveform created with famCreateFamily
c = 2*a
d = 2*b
plot(c d)

If I look at the plot that shows up, it lumps c and d waveforms into the same plot labeled  "(2*nil)"

I found that if I have a family waveform created with a getData() statement assigned to a variable (e.g. 'o_fam') I can pull it apart by using the command:

o_vec = drGetWaveformYVec(o_fam).

If I then query the o_vec, I can get something like:

o_vec->??
(units "" expression
    getData("/Q1C" ?resultsDir "/sim/fefan/bicmos8hp/beck_ma/TB_npn_distortion/spectre/__mod_VCB_tran_" ?result "tran")
)

If I try to pull-appart the family waveforms created with the famCreateFamily functions, I only get 'nil' when I query the y vector.

My assumption is that viva is looking at the expression embedded in the y vector and is grouping and labeling the waveforms based upon this.  

My question:
Is there a way to set the expression of a family waveform y-axis vector?