  We present a study of the anisotropy of the dielectric, magnetic and
magnetoelastic properties of the multiferroic clinopyroxene NaFeGe$_2$O$_6$.
Pyroelectric currents, dielectric constants and magnetic susceptibilities as
well as the thermal expansion and the magnetostriction were examined on large
synthetic single crystals of NaFeGe$_2$O$_6$. The spontaneous electric
polarization detected below $T_{\rm C}\simeq 11.6$ K in an
antiferromagnetically ordered state ($T_{\rm N}\simeq 13$ K) is mainly lying
within the $ac$ plane with a small component along $b$, indicating a triclinic
symmetry of the multiferroic phase of NaFeGe$_2$O$_6$. The electric
polarization can be strongly modified by applying magnetic fields along
different directions. We derive detailed magnetic-field versus temperature
phase diagrams and identify three multiferroic low-temperature phases, which
are separated by a non-ferroelectric, antiferromagnetically ordered state from
the paramagnetic high-temperature phase.
