Fix the "No visible binding" CRAN checks (#1504)

* fix cran check

* change required R version because of utils::globalVariables

R-package/R/xgb.ggplot.R

@@ -131,5 +131,5 @@ multiplot <- function(..., cols = 1) {
 
 globalVariables(c(
   "Cluster", "ggplot", "aes", "geom_bar", "coord_flip", "xlab", "ylab", "ggtitle", "theme",
-  "element_blank", "element_text"
+  "element_blank", "element_text", "V1", "Weight"
 ))

R-package/R/xgb.plot.deepness.R

@@ -144,6 +144,6 @@ get.leaf.depth <- function(dt_tree) {
 # They are mainly column names inferred by Data.table...
 globalVariables(
   c(
-    ".N", "N", "Depth", "Quality", "Cover", "Tree", "ID", "Yes", "No", "Feature"
+    ".N", "N", "Depth", "Quality", "Cover", "Tree", "ID", "Yes", "No", "Feature", "Leaf", "Weight"
   )
 )

R-package/R/xgboost.R

@@ -84,6 +84,7 @@ NULL
 #' @importFrom Matrix sparse.model.matrix
 #' @importFrom Matrix sparseVector
 #' @importFrom data.table data.table
+#' @importFrom data.table is.data.table
 #' @importFrom data.table as.data.table
 #' @importFrom data.table :=
 #' @importFrom data.table rbindlist
@@ -98,6 +99,12 @@ NULL
 #' @importFrom stringi stri_split_regex
 #' @importFrom utils object.size str tail
 #' @importFrom stats predict
+#' @importFrom stats median
+#' @importFrom utils head
+#' @importFrom graphics barplot
+#' @importFrom graphics grid
+#' @importFrom graphics par
+#' @importFrom graphics title
 #' 
 #' @import methods
 #' @useDynLib xgboost