[R-package] improve safety of index selection in plotting (#5485)

R-package/R/lgb.plot.importance.R

@@ -79,7 +79,7 @@ lgb.plot.importance <- function(tree_imp,
     )
   )
 
-  tree_imp[.N:1L,
+  tree_imp[rev(seq_len(.N)),
            graphics::barplot(
                height = get(measure)
                , names.arg = Feature

R-package/R/lgb.plot.interpretation.R

@@ -146,7 +146,7 @@ multiple.tree.plot.interpretation <- function(tree_interpretation,
   # create plot
   tree_interpretation[abs(Contribution) > 0.0, bar_color := "firebrick"]
   tree_interpretation[Contribution == 0.0, bar_color := "steelblue"]
-  tree_interpretation[.N:1L,
+  tree_interpretation[rev(seq_len(.N)),
                       graphics::barplot(
                           height = Contribution
                           , names.arg = Feature