A numerical code has been developed for modelling the asymmetries in three spectral lines (K I 7699, Fe I 5576 and Fe I 5635A) formed at different photospheric heights and observed in plage regions near the solar disk center. The aim of this work was to reproduce the absolute line bisectors obtained from low resolution observations. The mere insertion of a flux tube in a model of quiet granulation, without mutual interaction, has proved insufficient to explain many of the observational results. Therefore, several models of abnormal granulation close to the tube, representing the influence of the magnetic field, are considered. The code consists of two computation modules, one representing a model of the unperturbed photosphere, and the other including a flux tube inserted in a model of altered granulation; various combinations of both computation modules allow different values of the magnetic filling factor to be simulated. The influence of other physical parameters and observational aspects on the bisector is also discussed, e.g. waves, microturbulence, defective time or spatial averages, etc. The proposed model reproduces the observed variation in the shape and in the absolute shift of the bisector when the filling factor increases. Likewise, the computed spatial distribution of intensity in the continuum fits many of the photometric results reported in the literature for plage regions.