* add SHAP summary plot using ggplot2 * Update xgb.plot.shap * Update example in xgb.plot.shap documentation * update logic, add tests * whitespace fixes * whitespace fixes for test_helpers * namespace for sd function * explicitly declare variables that are automatically evaluated by data.table * Fix R lint Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>