Recently introduced exploratory spatial data analysis (ESDA) techniques provide social scientists with a new set of tools for distinguishing between random and nonrandom spatial patterns of events. Existing ESDA measures, however, are static and do not permit comparisons of distributions of events in the same space but across different time periods. One ESDA method--the Moran scatterplot --has special heuristic value because it visually displays local spatial relationships between each spatial unit and its neighbors. The model distinguishes between contagious diffusion between adjoining units and hierarchical diffusion that spreads broadly through commonly shared influences. The study applied the method to homicide data, looking for evidence of spatial diffusion of youth-gang homicides across neighborhoods in a city. Contagious diffusion between neighboring census tracts was evident only during the year of peak growth in total homicides, when high local rates of youth-gang homicides were followed by significant increases in neighboring youth-nongang rates. This pattern was consistent with a spread of homicides from gang youth to nongang youth. Otherwise, increases in both youth-gang and youth-nongang homicides generally occur simultaneously in non-neighboring tracts. Figures, notes, tables, references