How to plot input reflection coefficient in dB scale using Matlab
To do it numerically as others have commented use 10Log10(x). If you wish to plot with logarithmic scaling matlab has options such as log-log (logarithmic on both axis) or semilog (logarithmic on one axis). I think semilog is what your after as I think it would make sense to have frequency in logarithmic scaling and reflection coefficient linear.

some suggested reading:
MATLAB Log scaling
MATLAB Semilog