Provide element index with label_formatter

demo/src/plot_demo.rs

@@ -8,8 +8,8 @@ use egui::{
 
 use egui_plot::{
     Arrows, AxisHints, Bar, BarChart, BoxElem, BoxPlot, BoxSpread, CoordinatesFormatter, Corner,
-    GridInput, GridMark, HLine, Legend, Line, LineStyle, MarkerShape, Plot, PlotImage, PlotPoint,
-    PlotPoints, PlotResponse, Points, Polygon, Text, VLine,
+    GridInput, GridMark, HLine, HoverPosition, Legend, Line, LineStyle, MarkerShape, Plot,
+    PlotImage, PlotPoint, PlotPoints, PlotResponse, Points, Polygon, Text, VLine,
 };
 
 // ----------------------------------------------------------------------------
@@ -636,14 +636,19 @@ impl CustomAxesDemo {
             }
         };
 
-        let label_fmt = |_s: &str, val: &PlotPoint| {
-            format!(
+        let label_fmt = |position: &HoverPosition<'_>| match position {
+            HoverPosition::NearDataPoint {
+                plot_name: _,
+                position,
+                index: _,
+            } => 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)
-            )
+                d = day(position.x),
+                h = hour(position.x),
+                m = minute(position.x),
+                p = percent(position.y)
+            )),
+            HoverPosition::Elsewhere { position: _ } => None,
         };
 
         ui.label("Zoom in on the X-axis to see hours and minutes");
@@ -669,7 +674,7 @@ impl CustomAxesDemo {
             .custom_x_axes(x_axes)
             .custom_y_axes(y_axes)
             .x_grid_spacer(Self::x_grid)
-            .label_formatter(label_fmt)
+            .enumerated_label_formatter(label_fmt)
             .show(ui, |plot_ui| {
                 plot_ui.line(Self::logistic_fn());
             })

egui_plot/src/items/mod.rs

@@ -14,7 +14,9 @@ use egui::{
 use emath::Float as _;
 use rect_elem::{RectElement, highlighted_color};
 
-use super::{Cursor, LabelFormatter, PlotBounds, PlotTransform};
+use crate::HoverPosition;
+
+use super::{Cursor, NewLabelFormatter, PlotBounds, PlotTransform};
 
 pub use bar::Bar;
 pub use box_elem::{BoxElem, BoxSpread};
@@ -161,7 +163,7 @@ pub trait PlotItem {
         shapes: &mut Vec<Shape>,
         cursors: &mut Vec<Cursor>,
         plot: &PlotConfig<'_>,
-        label_formatter: &LabelFormatter<'_>,
+        label_formatter: &NewLabelFormatter<'_>,
     ) {
         let points = match self.geometry() {
             PlotGeometry::Points(points) => points,
@@ -187,7 +189,7 @@ pub trait PlotItem {
         rulers_and_tooltip_at_value(
             plot_area_response,
             value,
-            self.name(),
+            Some((self.name(), elem.index)),
             plot,
             cursors,
             label_formatter,
@@ -1441,7 +1443,7 @@ impl PlotItem for BarChart {
         shapes: &mut Vec<Shape>,
         cursors: &mut Vec<Cursor>,
         plot: &PlotConfig<'_>,
-        _: &LabelFormatter<'_>,
+        _: &NewLabelFormatter<'_>,
     ) {
         let bar = &self.bars[elem.index];
 
@@ -1568,7 +1570,7 @@ impl PlotItem for BoxPlot {
         shapes: &mut Vec<Shape>,
         cursors: &mut Vec<Cursor>,
         plot: &PlotConfig<'_>,
-        _: &LabelFormatter<'_>,
+        _: &NewLabelFormatter<'_>,
     ) {
         let box_plot = &self.boxes[elem.index];
 
@@ -1690,14 +1692,15 @@ fn add_rulers_and_text(
 /// and a label describing the coordinate.
 ///
 /// `value` is used to for text displaying X/Y coordinates.
+/// `nearest_point` contains the nearest point from a plot with its name and index.
 #[allow(clippy::too_many_arguments)]
 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: &LabelFormatter<'_>,
+    label_formatter: &NewLabelFormatter<'_>,
 ) {
     if plot.show_x {
         cursors.push(Cursor::Vertical { x: value.x });
@@ -1707,17 +1710,25 @@ pub(super) fn rulers_and_tooltip_at_value(
     }
 
     let text = if let Some(custom_label) = label_formatter {
-        custom_label(name, &value)
+        let hover_position = match nearest_point {
+            Some((name, index)) => HoverPosition::NearDataPoint {
+                plot_name: name,
+                position: value,
+                index: index,
+            },
+            None => HoverPosition::Elsewhere { position: value },
+        };
+        custom_label(&hover_position)
     } else {
-        let prefix = if name.is_empty() {
-            String::new()
-        } else {
+        let prefix = if let Some((name, _)) = nearest_point {
             format!("{name}\n")
+        } else {
+            String::new()
         };
         let scale = plot.transform.dvalue_dpos();
         let x_decimals = ((-scale[0].abs().log10()).ceil().at_least(0.0) as usize).clamp(1, 6);
         let y_decimals = ((-scale[1].abs().log10()).ceil().at_least(0.0) as usize).clamp(1, 6);
-        if plot.show_x && plot.show_y {
+        let result = if plot.show_x && plot.show_y {
             format!(
                 "{}x = {:.*}\ny = {:.*}",
                 prefix, x_decimals, value.x, y_decimals, value.y
@@ -1728,25 +1739,28 @@ pub(super) fn rulers_and_tooltip_at_value(
             format!("{}y = {:.*}", prefix, y_decimals, value.y)
         } else {
             unreachable!()
-        }
+        };
+        Some(result)
     };
 
-    // We show the tooltip as soon as we're hovering the plot area:
-    let mut tooltip = egui::Tooltip::always_open(
-        plot_area_response.ctx.clone(),
-        plot_area_response.layer_id,
-        plot_area_response.id,
-        PopupAnchor::Pointer,
-    );
+    if let Some(text) = text {
+        // We show the tooltip as soon as we're hovering the plot area:
+        let mut tooltip = egui::Tooltip::always_open(
+            plot_area_response.ctx.clone(),
+            plot_area_response.layer_id,
+            plot_area_response.id,
+            PopupAnchor::Pointer,
+        );
 
-    let tooltip_width = plot_area_response.ctx.style().spacing.tooltip_width;
+        let tooltip_width = plot_area_response.ctx.style().spacing.tooltip_width;
 
-    tooltip.popup = tooltip.popup.width(tooltip_width);
+        tooltip.popup = tooltip.popup.width(tooltip_width);
 
-    tooltip.gap(12.0).show(|ui| {
-        ui.set_max_width(tooltip_width);
-        ui.label(text);
-    });
+        tooltip.gap(12.0).show(|ui| {
+            ui.set_max_width(tooltip_width);
+            ui.label(text);
+        });
+    }
 }
 
 fn find_closest_rect<'a, T>(

egui_plot/src/lib.rs

@@ -45,6 +45,9 @@ use legend::LegendWidget;
 type LabelFormatterFn<'a> = dyn Fn(&str, &PlotPoint) -> String + 'a;
 pub type LabelFormatter<'a> = Option<Box<LabelFormatterFn<'a>>>;
 
+type NewLabelFormatterFn<'a> = dyn Fn(&HoverPosition<'_>) -> Option<String> + 'a;
+pub type NewLabelFormatter<'a> = Option<Box<NewLabelFormatterFn<'a>>>;
+
 type GridSpacerFn<'a> = dyn Fn(GridInput) -> Vec<GridMark> + 'a;
 type GridSpacer<'a> = Box<GridSpacerFn<'a>>;
 
@@ -92,6 +95,23 @@ pub enum Cursor {
     Vertical { x: f64 },
 }
 
+/// 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,
+    },
+}
+
 /// Contains the cursors drawn for a plot widget in a single frame.
 #[derive(PartialEq, Clone)]
 struct PlotFrameCursors {
@@ -178,7 +198,7 @@ pub struct Plot<'a> {
 
     show_x: bool,
     show_y: bool,
-    label_formatter: LabelFormatter<'a>,
+    label_formatter: NewLabelFormatter<'a>,
     coordinates_formatter: Option<(Corner, CoordinatesFormatter<'a>)>,
     x_axes: Vec<AxisHints<'a>>, // default x axes
     y_axes: Vec<AxisHints<'a>>, // default y axes
@@ -425,6 +445,46 @@ impl<'a> Plot<'a> {
     pub fn label_formatter(
         mut self,
         label_formatter: impl Fn(&str, &PlotPoint) -> String + 'a,
+    ) -> Self {
+        let inner_box = Box::new(label_formatter);
+        self.label_formatter = Some(Box::new(move |position| {
+            Some(match position {
+                HoverPosition::NearDataPoint {
+                    plot_name,
+                    position,
+                    index: _,
+                } => inner_box(plot_name, &position),
+                HoverPosition::Elsewhere { position: _ } => "".to_owned(),
+            })
+        }));
+        self
+    }
+
+    /// Provide a function to customize the on-hover label for the x and y axis.
+    /// This is a generalized version of `label_formatter` that also provides the point's index,
+    /// and allows for the tooltip to be hidden conditionally by returning an Option<String>
+    ///
+    /// ```
+    /// # egui::__run_test_ui(|ui| {
+    /// use egui_plot::{Line, Plot, PlotPoints};
+    /// let sin: PlotPoints = (0..1000).map(|i| {
+    ///     let x = i as f64 * 0.01;
+    ///     [x, x.sin()]
+    /// }).collect();
+    /// let line = Line::new("sin", sin);
+    /// Plot::new("my_plot").view_aspect(2.0)
+    /// .enumerated_label_formatter(|context| {
+    ///     match context {
+    ///         Some((name, index)) => format!("{}: {:.*}%", name, 1, index),
+    ///         Elsewhere { ... } => None,
+    ///     }
+    /// })
+    /// .show(ui, |plot_ui| plot_ui.line(line));
+    /// # });
+    /// ```
+    pub fn enumerated_label_formatter(
+        mut self,
+        label_formatter: impl Fn(&HoverPosition<'_>) -> Option<String> + 'a,
     ) -> Self {
         self.label_formatter = Some(Box::new(label_formatter));
         self
@@ -1598,7 +1658,7 @@ struct PreparedPlot<'a> {
     items: Vec<Box<dyn PlotItem + 'a>>,
     show_x: bool,
     show_y: bool,
-    label_formatter: LabelFormatter<'a>,
+    label_formatter: NewLabelFormatter<'a>,
     coordinates_formatter: Option<(Corner, CoordinatesFormatter<'a>)>,
     // axis_formatters: [AxisFormatter; 2],
     transform: PlotTransform,
@@ -1858,7 +1918,7 @@ impl PreparedPlot<'_> {
             items::rulers_and_tooltip_at_value(
                 plot_area_response,
                 value,
-                "",
+                None,
                 &plot,
                 &mut cursors,
                 label_formatter,