Apparatus and methods for measuring and analyzing blood flow in vessels in the bodies of living subjects are provided. The true velocity vector, or a projection thereof onto a scan plane, for blood flowing at a location in a vessel is determined from multiple Doppler velocity components. The true velocity vector is displayed as a directional marker on an image comprising the vessel. The true velocity magnitude and angle, and scalar blood flow characteristics derived therefrom, are plotted as a functions of time and/or position across the depiction of the blood vessel in the image. Time intervals for which blood flow characteristic data is displayed and/or determined may be user configurable.