diff --git a/charming/src/element/formatter.rs b/charming/src/element/formatter.rs
index 3b55904..c9e6f55 100644
--- a/charming/src/element/formatter.rs
+++ b/charming/src/element/formatter.rs
@@ -1,12 +1,63 @@
+use super::RawString;
 use serde::Serialize;
 
-use super::RawString;
+#[derive(Serialize)]
+#[serde(transparent)]
+pub struct FormatterFunction {
+    #[cfg(not(target_arch = "wasm32"))]
+    pub value: RawString,
+    #[cfg(target_arch = "wasm32")]
+    pub value: web_sys::js_sys::Function,
+}
+
+impl FormatterFunction {
+    #[cfg(not(target_arch = "wasm32"))]
+    pub fn new_no_args(body: &str) -> FormatterFunction {
+        FormatterFunction {
+            value: RawString::from(format!("function() {{ {} }}", body)),
+        }
+    }
+
+    #[cfg(target_arch = "wasm32")]
+    pub fn new_no_args(body: &str) -> FormatterFunction {
+        FormatterFunction {
+            value: web_sys::js_sys::Function::new_no_args(body),
+        }
+    }
+
+    #[cfg(not(target_arch = "wasm32"))]
+    pub fn new_with_args(args: &str, body: &str) -> FormatterFunction {
+        FormatterFunction {
+            value: RawString::from(format!("function({}) {{ {} }}", args, body)),
+        }
+    }
+
+    #[cfg(target_arch = "wasm32")]
+    pub fn new_with_args(args: &str, body: &str) -> FormatterFunction {
+        FormatterFunction {
+            value: web_sys::js_sys::Function::new_with_args(args, body),
+        }
+    }
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+
+impl<S> From<S> for FormatterFunction
+where
+    S: Into<String>,
+{
+    fn from(s: S) -> Self {
+        FormatterFunction {
+            value: RawString::from(s),
+        }
+    }
+}
 
 #[derive(Serialize)]
 #[serde(untagged)]
 pub enum Formatter {
     String(String),
-    Function(RawString),
+    Function(FormatterFunction),
 }
 
 impl From<&str> for Formatter {
@@ -15,8 +66,15 @@ impl From<&str> for Formatter {
     }
 }
 
+#[cfg(not(target_arch = "wasm32"))]
 impl From<RawString> for Formatter {
     fn from(s: RawString) -> Self {
-        Formatter::Function(s)
+        Formatter::Function(FormatterFunction { value: s })
+    }
+}
+
+impl From<FormatterFunction> for Formatter {
+    fn from(f: FormatterFunction) -> Self {
+        Formatter::Function(f)
     }
 }
diff --git a/charming/src/element/symbol_size.rs b/charming/src/element/symbol_size.rs
index 242c72a..27f540b 100644
--- a/charming/src/element/symbol_size.rs
+++ b/charming/src/element/symbol_size.rs
@@ -1,12 +1,12 @@
 use serde::Serialize;
 
-use super::RawString;
+use super::{FormatterFunction, RawString};
 
 #[derive(Serialize)]
 #[serde(untagged)]
 pub enum SymbolSize {
     Number(f64),
-    Function(RawString),
+    Function(FormatterFunction),
 }
 
 impl From<i64> for SymbolSize {
@@ -21,8 +21,17 @@ impl From<f64> for SymbolSize {
     }
 }
 
+#[cfg(not(target_arch = "wasm32"))]
 impl From<&str> for SymbolSize {
     fn from(s: &str) -> Self {
-        SymbolSize::Function(RawString::from(s))
+        SymbolSize::Function(FormatterFunction {
+            value: RawString::from(s),
+        })
+    }
+}
+
+impl From<FormatterFunction> for SymbolSize {
+    fn from(f: FormatterFunction) -> Self {
+        SymbolSize::Function(f)
     }
 }
diff --git a/charming/src/element/tooltip.rs b/charming/src/element/tooltip.rs
index 7102386..a006a1d 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