Plot a 'grouped' univariate histogram

Function HISTG ('grouped histogram') adapts Matlab's HIST to cases where data vector includes values from several observed groups, and lets one color-code group membership on the overall histogram. Separate histograms are constructed for all x-groups (each identified by a distinct finite value of a class-assignment variable c), and vertically stacked. HISTG offers display options (x-percentile cut-offs, absolute axis limits, bin size, marker line) implemented in HISTF, linked to below. (Note: when range of x is limited with [pmin, pmax], no adjustment for truncation is made. The cut-offs are meant to exclude 'extreme', low-mass values).