making an overplot of a curve function

Using Isis I am familiar with the commands to subtract one spectrum from
another, and then plot the residual flux.
eg where one spectrum is data and the other model

d = get_data_counts(1);
m = get_model_counts(1); % continuum model only

d.value -= m.value; % subtract the continuum model

id = define_counts (d); % define a new continuum-
% subtracted data set.

My present issue is a case where one spectrum (A) is considered to have had
absorption occur over a considerable wavelength region, and the other (B)
has not; I want to calculate 

   tau =   ln (fluxB / fluxA) 

and  plot this value as an optical depth, with the x-axis in angstroms. 

I would ideally like to overplot the optical depth curve. 

Any suggestions would be greatly appreciated.

Greg


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