Provide element index with label_formatter

egui_plot/src/items/mod.rs

@@ -40,6 +40,7 @@ pub use crate::items::polygon::Polygon;
 pub use crate::items::series::Line;
 pub use crate::items::span::Span;
 pub use crate::items::text::Text;
+use crate::label::HoverPosition;
 use crate::label::LabelFormatter;
 use crate::rect_elem::RectElement;
 
@@ -198,7 +199,14 @@ pub trait PlotItem {
         let pointer = plot.transform.position_from_point(&value);
         shapes.push(Shape::circle_filled(pointer, 3.0, line_color));
 
-        rulers_and_tooltip_at_value(plot_area_response, value, self.name(), plot, cursors, label_formatter);
+        rulers_and_tooltip_at_value(
+            plot_area_response,
+            value,
+            Some((self.name(), elem.index)),
+            plot,
+            cursors,
+            label_formatter,
+        );
     }
 }
 
@@ -272,7 +280,7 @@ fn add_rulers_and_text(
 pub(super) fn rulers_and_tooltip_at_value(
     plot_area_response: &egui::Response,
     value: PlotPoint,
-    name: &str,
+    nearest_point: Option<(&str, usize)>,
     plot: &PlotConfig<'_>,
     cursors: &mut Vec<Cursor>,
     label_formatter: &Option<LabelFormatter<'_>>,
@@ -292,10 +300,18 @@ pub(super) fn rulers_and_tooltip_at_value(
         return;
     };
 
-    let text = custom_label(name, &value);
-    if text.is_empty() {
+    let hover_position = match nearest_point {
+        Some((name, index)) => HoverPosition::NearDataPoint {
+            plot_name: name,
+            position: value,
+            index,
+        },
+        None => HoverPosition::Elsewhere { position: value },
+    };
+
+    let Some(text) = custom_label(&hover_position) else {
         return;
-    }
+    };
 
     // We show the tooltip as soon as we're hovering the plot area:
     let mut tooltip = egui::Tooltip::always_open(

egui_plot/src/label.rs

@@ -16,18 +16,40 @@ pub fn format_number(number: f64, num_decimals: usize) -> String {
     }
 }
 
-type LabelFormatterFn<'a> = dyn Fn(&str, &PlotPoint) -> String + 'a;
+type LabelFormatterFn<'a> = dyn Fn(&HoverPosition<'_>) -> Option<String> + 'a;
 
 /// Optional label formatter function for customizing hover labels.
 pub type LabelFormatter<'a> = Box<LabelFormatterFn<'a>>;
 
+/// Indicates the position of the cursor in a plot for hover purposes.
+#[derive(Copy, Clone, PartialEq)]
+pub enum HoverPosition<'a> {
+    NearDataPoint {
+        /// The name of the plot whose data point is nearest to the cursor
+        plot_name: &'a str,
+
+        /// The position of the nearest data point
+        position: PlotPoint,
+
+        /// The index of the nearest data point in its plot
+        index: usize,
+    },
+    Elsewhere {
+        /// The position in the plot over which the cursor hovers
+        position: PlotPoint,
+    },
+}
+
 /// Default label formatter that shows the x and y coordinates with 3 decimal
 /// places.
-pub fn default_label_formatter(name: &str, value: &PlotPoint) -> String {
-    let prefix = if name.is_empty() {
-        String::new()
-    } else {
-        format!("{name}\n")
-    };
-    format!("{}x = {:.3}\ny = {:.3}", prefix, value.x, value.y)
+#[expect(clippy::unnecessary_wraps)]
+pub fn default_label_formatter(pos: &HoverPosition<'_>) -> Option<String> {
+    Some(match pos {
+        HoverPosition::NearDataPoint {
+            plot_name,
+            position,
+            index: _,
+        } => format!("{}\nx = {:.3}\ny = {:.3}", plot_name, position.x, position.y),
+        HoverPosition::Elsewhere { position } => format!("x = {:.3}\ny = {:.3}", position.x, position.y),
+    })
 }

egui_plot/src/lib.rs

@@ -61,6 +61,7 @@ pub use crate::items::Polygon;
 pub use crate::items::Span;
 pub use crate::items::Text;
 pub use crate::items::VLine;
+pub use crate::label::HoverPosition;
 pub use crate::label::LabelFormatter;
 pub use crate::label::default_label_formatter;
 pub use crate::label::format_number;

egui_plot/src/plot.rs

@@ -47,6 +47,7 @@ use crate::items::PlotItem;
 use crate::items::Span;
 use crate::items::horizontal_line;
 use crate::items::vertical_line;
+use crate::label::HoverPosition;
 use crate::label::LabelFormatter;
 use crate::memory::PlotMemory;
 use crate::overlays::CoordinatesFormatter;
@@ -396,9 +397,7 @@ impl<'a> Plot<'a> {
     ///
     /// ```
     /// # egui::__run_test_ui(|ui| {
-    /// use egui_plot::Line;
-    /// use egui_plot::Plot;
-    /// use egui_plot::PlotPoints;
+    /// use egui_plot::{HoverPosition, Line, Plot, PlotPoints};
     /// let sin: PlotPoints = (0..1000)
     ///     .map(|i| {
     ///         let x = i as f64 * 0.01;
@@ -408,18 +407,17 @@ impl<'a> Plot<'a> {
     /// let line = Line::new("sin", sin);
     /// Plot::new("my_plot")
     ///     .view_aspect(2.0)
-    ///     .label_formatter(|name, value| {
-    ///         if !name.is_empty() {
-    ///             format!("{}: {:.*}%", name, 1, value.y)
-    ///         } else {
-    ///             "".to_owned()
+    ///     .label_formatter(|pos| match pos {
+    ///         HoverPosition::NearDataPoint { plot_name, position, .. } if !plot_name.is_empty() => {
+    ///             Some(format!("{}: {:.*}%", plot_name, 1, position.y))
     ///         }
+    ///         _ => None,
     ///     })
     ///     .show(ui, |plot_ui| plot_ui.line(line));
     /// # });
     /// ```
     #[inline]
-    pub fn label_formatter(mut self, label_formatter: impl Fn(&str, &PlotPoint) -> String + 'a) -> Self {
+    pub fn label_formatter(mut self, label_formatter: impl Fn(&HoverPosition<'_>) -> Option<String> + 'a) -> Self {
         self.label_formatter = Some(Box::new(label_formatter));
         self
     }
@@ -1601,7 +1599,7 @@ impl<'a> Plot<'a> {
             items::rulers_and_tooltip_at_value(
                 &plot_ui.response,
                 value,
-                "",
+                None,
                 &plot,
                 &mut cursors,
                 &self.label_formatter,

examples/custom_axes/src/app.rs

@@ -5,9 +5,9 @@ use eframe::egui::Response;
 use egui_plot::AxisHints;
 use egui_plot::GridInput;
 use egui_plot::GridMark;
+use egui_plot::HoverPosition;
 use egui_plot::Line;
 use egui_plot::Plot;
-use egui_plot::PlotPoint;
 use egui_plot::PlotPoints;
 
 #[derive(Default)]
@@ -101,14 +101,22 @@ impl CustomAxesExample {
             }
         };
 
-        let label_fmt = |_s: &str, val: &PlotPoint| {
-            format!(
+        let label_fmt = |pos: &HoverPosition<'_>| {
+            let val = match pos {
+                HoverPosition::NearDataPoint {
+                    plot_name: _,
+                    position,
+                    index: _,
+                }
+                | HoverPosition::Elsewhere { position } => position,
+            };
+            Some(format!(
                 "Day {d}, {h}:{m:02}\n{p:.2}%",
                 d = day(val.x),
                 h = hour(val.x),
                 m = minute(val.x),
                 p = percent(val.y)
-            )
+            ))
         };
 
         let x_axes = vec![