  We present a new quasi-probability distribution function for ensembles of
spin-half particles or qubits that has many properties in common with Wigner's
original function for systems of continuous variables. We show that this
function provides clear and intuitive graphical representation of a wide
variety of states, including Fock states, spin-coherent states, squeezed
states, superpositions and statistical mixtures. Unlike previous attempts to
represent ensembles of spins/qubits, this distribution is capable of
simultaneously representing several angular momentum shells.
