diff --git a/charming/src/element/tooltip.rs b/charming/src/element/tooltip.rs
index f1237cf..df9817e 100644
--- a/charming/src/element/tooltip.rs
+++ b/charming/src/element/tooltip.rs
@@ -36,6 +36,9 @@ pub struct Tooltip {
     #[serde(skip_serializing_if = "Option::is_none")]
     formatter: Option<Formatter>,
 
+    #[serde(skip_serializing_if = "Option::is_none")]
+    value_formatter: Option<Formatter>,
+
     #[serde(skip_serializing_if = "Option::is_none")]
     position: Option<String>,
 
@@ -65,6 +68,7 @@ impl Tooltip {
             trigger_on: None,
             axis_pointer: None,
             formatter: None,
+            value_formatter: None,
             position: None,
             padding: None,
             background_color: None,
@@ -93,6 +97,11 @@ impl Tooltip {
         self
     }
 
+    pub fn value_formatter<F: Into<Formatter>>(mut self, value_formatter: F) -> Self {
+        self.value_formatter = Some(value_formatter.into());
+        self
+    }
+
     pub fn position<S: Into<String>>(mut self, position: S) -> Self {
         self.position = Some(position.into());
         self
diff --git a/gallery/src/line/distribution_of_electricity.rs b/gallery/src/line/distribution_of_electricity.rs
index 45ceae3..d29f7e0 100644
--- a/gallery/src/line/distribution_of_electricity.rs
+++ b/gallery/src/line/distribution_of_electricity.rs
@@ -1,8 +1,8 @@
 use charming::{
     component::{Axis, Feature, SaveAsImage, Title, Toolbox, VisualMap, VisualMapPiece},
     element::{
-        AxisLabel, AxisPointer, AxisPointerType, AxisType, ItemStyle, MarkArea, MarkAreaData,
-        Tooltip, Trigger,
+        AxisLabel, AxisPointer, AxisPointerType, AxisType, Formatter, ItemStyle, MarkArea,
+        MarkAreaData, Tooltip, Trigger,
     },
     series::Line,
     Chart,
@@ -18,7 +18,8 @@ pub fn chart() -> Chart {
         .tooltip(
             Tooltip::new()
                 .trigger(Trigger::Axis)
-                .axis_pointer(AxisPointer::new().type_(AxisPointerType::Cross)),
+                .axis_pointer(AxisPointer::new().type_(AxisPointerType::Cross))
+                .value_formatter(Formatter::Function("value => value + 'W'".into())),
         )
         .toolbox(
             Toolbox::new()