[python-package] fix mypy errors in `plotting.py` (#4838)

* [python-package] fix mypy errors in plotting.py

* empty commit

python-package/lightgbm/plotting.py

@@ -236,12 +236,12 @@ def plot_split_value_histogram(
     elif not isinstance(booster, Booster):
         raise TypeError('booster must be Booster or LGBMModel.')
 
-    hist, bins = booster.get_split_value_histogram(feature=feature, bins=bins, xgboost_style=False)
+    hist, split_bins = booster.get_split_value_histogram(feature=feature, bins=bins, xgboost_style=False)
     if np.count_nonzero(hist) == 0:
         raise ValueError('Cannot plot split value histogram, '
                          f'because feature {feature} was not used in splitting')
-    width = width_coef * (bins[1] - bins[0])
-    centred = (bins[:-1] + bins[1:]) / 2
+    width = width_coef * (split_bins[1] - split_bins[0])
+    centred = (split_bins[:-1] + split_bins[1:]) / 2
 
     if ax is None:
         if figsize is not None:
@@ -253,8 +253,8 @@ def plot_split_value_histogram(
     if xlim is not None:
         _check_not_tuple_of_2_elements(xlim, 'xlim')
     else:
-        range_result = bins[-1] - bins[0]
-        xlim = (bins[0] - range_result * 0.2, bins[-1] + range_result * 0.2)
+        range_result = split_bins[-1] - split_bins[0]
+        xlim = (split_bins[0] - range_result * 0.2, split_bins[-1] + range_result * 0.2)
     ax.set_xlim(xlim)
 
     ax.yaxis.set_major_locator(MaxNLocator(integer=True))
@@ -358,13 +358,13 @@ def plot_metric(
         _, ax = plt.subplots(1, 1, figsize=figsize, dpi=dpi)
 
     if dataset_names is None:
-        dataset_names = iter(eval_results.keys())
+        dataset_names_iter = iter(eval_results.keys())
     elif not isinstance(dataset_names, (list, tuple, set)) or not dataset_names:
         raise ValueError('dataset_names should be iterable and cannot be empty')
     else:
-        dataset_names = iter(dataset_names)
+        dataset_names_iter = iter(dataset_names)
 
-    name = next(dataset_names)  # take one as sample
+    name = next(dataset_names_iter)  # take one as sample
     metrics_for_one = eval_results[name]
     num_metric = len(metrics_for_one)
     if metric is None:
@@ -381,7 +381,7 @@ def plot_metric(
     x_ = range(num_iteration)
     ax.plot(x_, results, label=name)
 
-    for name in dataset_names:
+    for name in dataset_names_iter:
         metrics_for_one = eval_results[name]
         results = metrics_for_one[metric]
         max_result = max(max(results), max_result)