  A pseudospin model for description of the influence of the electric field,
confined to the plane of sublattice polarization, on the two-dimensional
squaric acid antiferroelectrics is developed. The system behavior is analyzed
in terms of the parameters of ferroelectric and antiferroelectric ordering, as
well as the non-collinearity angle $\theta$, which is the angle between
sublattice polarizations. The temperature-electric field $T-E_1$ phase diagram
is constructed. Most of the field-induced transitions are found to be
associated with polarization rotation. The observed phases include the
collinear ferroelectric and non-collinear ferrielectric with almost
antiparallel and perpendicular polarizations of the sublattices, respectively.
The diagram also contains two regions, where the non-collinearity angle varies
continuously between, nominally, 0 and 90$^\circ$ and between 90$^\circ$ and
180$^\circ$. The first and second order transition lines, supercritical lines,
critical end points, and critical points on the $T-E_1$ phase diagram are
detected.
