  We present a frame-invariant method for detecting coherent structures from
Lagrangian flow trajectories that can be sparse in number, as is the case in
many fluid mechanics applications of practical interest. The method, based on
principles used in graph coloring and spectral graph drawing algorithms,
examines a measure of the kinematic dissimilarity of all pairs of fluid
trajectories, either measured experimentally, e.g. using particle tracking
velocimetry; or numerically, by advecting fluid particles in the Eulerian
velocity field. Coherence is assigned to groups of particles whose kinematics
remain similar throughout the time interval for which trajectory data is
available, regardless of their physical proximity to one another. Through the
use of several analytical and experimental validation cases, this algorithm is
shown to robustly detect coherent structures using significantly less flow data
than is required by existing spectral graph theory methods.
