Provides two functions: Group_function() and Parallel_Tree(). Group_function() applies a given function (e.g., mean()) to input variable(s) by group across levels of a multilevel data structure, with additional data management options. Parallel_Tree() uses 'ggplot2' to create parallel coordinate plots (technically a facsimile of parallel coordinate plots in a Cartesian coordinate system). Used in combination, these functions can create parallel tree plots, a variant of parallel coordinate plots useful for visualizing multilevel data.