diff --git a/ckanext/charts/chart_builders/plotly.py b/ckanext/charts/chart_builders/plotly.py
index c38c1b9..b4962a0 100644
--- a/ckanext/charts/chart_builders/plotly.py
+++ b/ckanext/charts/chart_builders/plotly.py
@@ -78,32 +78,30 @@ class PlotlyLineBuilder(PlotlyBuilder):
     def to_json(self) -> Any:
         return self.build_line_chart()
 
-    def _split_data_by_year(self) -> dict[str, Any]:
+    def _split_data_by_year(self) -> None:
         """
         Prepare data for a line chart. It splits the data by year stated
         in the date format column which is used for x-axis.
         """
-        if len(self.settings["years"]) > 1:
-            self.df.drop_duplicates(subset=[self.settings["x"]], inplace=True)
-            self.df = self.df[[self.settings["x"], self.settings["y"][0]]]
-            self.df["year"] = pd.to_datetime(self.df[self.settings["x"]]).dt.year
-
-            self.df = self.df.pivot(
-                index=self.settings["x"],
-                columns="year",
-                values=self.settings["y"][0],
-            )
+        if len(self.settings["years"]) <= 1:
+            return
 
-            self.settings["y"] = self.df.columns.to_list()
-            self.df[self.settings["x"]] = self.df.index
-            self.df[self.settings["x"]] = pd.to_datetime(
-                self.df[self.settings["x"]],
-                unit="ns",
-            ).dt.strftime("%m-%d %H:%M")
+        self.df.drop_duplicates(subset=[self.settings["x"]], inplace=True)
+        self.df = self.df[[self.settings["x"], self.settings["y"][0]]]
+        self.df["year"] = pd.to_datetime(self.df[self.settings["x"]]).dt.year
+
+        self.df = self.df.pivot(
+            index=self.settings["x"], columns="year", values=self.settings["y"][0]
+        )
 
-        return self
+        self.settings["y"] = self.df.columns.to_list()
+        self.df[self.settings["x"]] = self.df.index
+        self.df[self.settings["x"]] = pd.to_datetime(
+            self.df[self.settings["x"]],
+            unit="ns",
+        ).dt.strftime("%m-%d %H:%M")
 
-    def _skip_null_values(self, column: str) -> tuple[Any]:
+    def _skip_null_values(self, column: str) -> tuple[Any, Any]:
         """
         Return values for x-axis and y-axis after removing missing values.
         """
@@ -124,7 +122,9 @@ def _skip_null_values(self, column: str) -> tuple[Any]:
 
         return x, y
 
-    def _break_chart_by_missing_data(self, df: DataFrame, column: str) -> tuple[Any]:
+    def _break_chart_by_missing_data(
+        self, df: DataFrame, column: str
+    ) -> tuple[Any, Any]:
         """
         Find gaps in date column and fill them with missing dates.
         """