  We consider a class of manifolds with torus boundary admitting bordered
Heegaard Floer homology of a particularly simple form, namely, the type D
structure may be described graphically by a disjoint union of loops. We develop
a calculus for studying bordered invariants of this form and, in particular,
provide a complete description of slopes giving rise to L-space Dehn fillings
as well as necessary and sufficient conditions for L-spaces resulting from
identifying two such manifolds along their boundaries. As an application, we
show that Seifert fibered spaces with torus boundary fall into this class,
leading to a proof that, among graph manifolds containing a single JSJ torus,
the property of being an L-space is equivalent to non-left-orderability of the
fundamental group and to the non-existence of a coorientable taut foliation.
