Abstract: To alleviate the deficiency of excessive edge crossing brought by random layout of categories in categorical data visualization—parallel sets,we propose a heuristic layout algorithm based on median,which optimise the layout order of categories such that visual clutter is eased.And then we utilize the improved parallel sets to analyze the global terrorism database(GTD).The experimental results show that the improved parallel sets can clearly express the association among the multi-categories in GTD,thereby assist users in analyzing the implicit information of various terrorist organizations,such as the behavior characteristics.Furthermore,the median-based heuristic is simple and has high efficiency,which is suitable for large data sets with many categorical attributes.