There are many ways to compute and plot the oxygen fugacity of the graphite-C-O-fluid buffer (CCO), the four easiest are:
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With the Program FLUIDS:

1) In FLUIDS choose equation of state #10 or #11. 

2) Answer yes to the tabulate properties prompt.

3) Choose temperature as the independent variable, specify X(O) = 1 (i.e., a C-O fluid), log[a_C] = 0 (i.e., C activity = 1), and the pressure, or range of pressures, of interest.

4) Plot the result using PSTABLE or perple_x_plot (MatLab).

This method accounts for the fact that the fluid is impure (a mixture of CO2 and CO) and corrects for the diamond/graphite transition. The disadvantage of this method is that the thermodynamic data is built into the program.
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With the Program FRENDLY:

1) Create (or uncomment) the make definition in the header of your preferred thermodynamic data file:

CCO_fo2 = -1 CO2 + 1 gph + 1 O2         | graphite CCO oxygen fugacity (or activity)
DQF = 0

2) Verify that the EoS flag for CO2 is 102 or 202 and that the EoS flag for O2 is 1, for more information on this modficiation refer to the warning in: 

www.perplex.ethz.ch/perplex/faq/how_to_compute_fO2_for_a_buffer_with_frendly.txt

3) run FRENDLY.

This method approximates the CCO fluid as pure CO2, usually an excellent approximation for geological conditions. A second make definition (involving diamond) would be required for CCO within the diamond stability field.
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With the Programs MEEMUM/VERTEX as a function of P,T:

1) Run BUILD to compute a phase diagram section for the C-O2 system, specify a molar amount of C that is greater than the molar amount of O2, this assures that the system will be saturated in graphite or diamond, answer yes to the include solution model prompt and select a generic hybrid fluid EoS solution model (e.g. COH_Fluid+).

2) Run MEEMUM/VERTEX to compute the chemical potential of oxygen (mu_O2) for the system. Use WERAMI to tabulate mu_O2.

3) The tabulated values of mu_O2 can then be converted to the fugacity of the O2 species by computing the Gibbs energy of the O2 species at the temperature of interest and the reference pressure (G0_O2[P0,T]) with FRENDLY, i.e., ln[f_O2] = (mu_O2(P,T))-G0_O2(P0,T))/R/T.

Refer to perplex.ethz.ch/Perple_X_generic_hyrbid_fluid_EoS.html for information on how to manipulate hybrid fluid EoS solution models. 

This method permits the user complete control over the choice of thermodynamic data, equations of state, and speciation (e.g., monatomic O can be added to the speciation model if desired). 
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With the Programs MEEMUM/VERTEX as a function of f_O2:

Refer to www.perplex.ethz.ch/perplex/faq/how_to_compute_CCO_in_VERTEX.zip

This method is relevant when it is desired to simultaneously the stability of a buffer assemblage with other phase relations as an explicit function of f_O2. 

There is no reason to use this method unless you are interested in the stability of the buffer assemblage relative to other phases.
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See also:

www.perplex.ethz.ch/perplex/faq/how_to_compute_fO2_for_a_buffer_with_frendly.txt
www.perplex.ethz.ch/perplex/faq/how_to_buffer_chemical_potentials.txt
www.perplex.ethz.ch/perplex/faq/calculate_fo2_for_a_buffer_frendly_dialog.txt