[R-package] added test on uncovered line of lgb.plot.importance() (#2903)

R-package/tests/testthat/test_lgb.plot.importance.R

@@ -17,11 +17,20 @@ test_that("lgb.plot.importance() should run without error for well-formed inputs
 
     # Check that there are no plots present before plotting
     expect_null(dev.list())
-    resDT <- lgb.plot.importance(
-        tree_imp = tree_imp
+
+    args_no_cex <- list(
+        "tree_imp" = tree_imp
         , top_n = 10L
         , measure = "Gain"
-        , cex = 0.75
+    )
+    args_cex <- args_no_cex
+    args_cex[["cex"]] <- 0.75
+
+    for (arg_list in list(args_no_cex, args_cex)) {
+
+        resDT <- do.call(
+            what = lgb.plot.importance
+            , args = arg_list
         )
 
         # Check that lgb.plot.importance() returns the data.table of the plotted data
@@ -34,4 +43,5 @@ test_that("lgb.plot.importance() should run without error for well-formed inputs
         # remove all plots
         dev.off()
         expect_null(dev.list())
+    }
 })