Add ability to customize the display of hover plot labels

egui/src/widgets/plot/items/mod.rs

@@ -7,7 +7,7 @@ use epaint::Mesh;
 use crate::util::float_ord::FloatOrd;
 use crate::*;
 
-use super::{PlotBounds, ScreenTransform};
+use super::{CustomLabelFunc, PlotBounds, ScreenTransform};
 use rect_elem::*;
 use values::*;
 
@@ -61,7 +61,13 @@ pub(super) trait PlotItem {
         }
     }
 
-    fn on_hover(&self, elem: ClosestElem, shapes: &mut Vec<Shape>, plot: &PlotConfig<'_>) {
+    fn on_hover(
+        &self,
+        elem: ClosestElem,
+        shapes: &mut Vec<Shape>,
+        plot: &PlotConfig<'_>,
+        custom_label_func: &CustomLabelFunc,
+    ) {
         let points = match self.geometry() {
             PlotGeometry::Points(points) => points,
             PlotGeometry::None => {
@@ -83,7 +89,7 @@ pub(super) trait PlotItem {
         let pointer = plot.transform.position_from_value(&value);
         shapes.push(Shape::circle_filled(pointer, 3.0, line_color));
 
-        rulers_at_value(pointer, value, self.name(), plot, shapes);
+        rulers_at_value(pointer, value, self.name(), plot, shapes, custom_label_func);
     }
 }
 
@@ -1365,7 +1371,13 @@ impl PlotItem for BarChart {
         find_closest_rect(&self.bars, point, transform)
     }
 
-    fn on_hover(&self, elem: ClosestElem, shapes: &mut Vec<Shape>, plot: &PlotConfig<'_>) {
+    fn on_hover(
+        &self,
+        elem: ClosestElem,
+        shapes: &mut Vec<Shape>,
+        plot: &PlotConfig<'_>,
+        _: &CustomLabelFunc,
+    ) {
         let bar = &self.bars[elem.index];
 
         bar.add_shapes(plot.transform, true, shapes);
@@ -1501,7 +1513,13 @@ impl PlotItem for BoxPlot {
         find_closest_rect(&self.boxes, point, transform)
     }
 
-    fn on_hover(&self, elem: ClosestElem, shapes: &mut Vec<Shape>, plot: &PlotConfig<'_>) {
+    fn on_hover(
+        &self,
+        elem: ClosestElem,
+        shapes: &mut Vec<Shape>,
+        plot: &PlotConfig<'_>,
+        _: &CustomLabelFunc,
+    ) {
         let box_plot = &self.boxes[elem.index];
 
         box_plot.add_shapes(plot.transform, true, shapes);
@@ -1619,6 +1637,7 @@ pub(super) fn rulers_at_value(
     name: &str,
     plot: &PlotConfig<'_>,
     shapes: &mut Vec<Shape>,
+    custom_label_func: &CustomLabelFunc,
 ) {
     let line_color = rulers_color(plot.ui);
     if plot.show_x {
@@ -1638,7 +1657,9 @@ pub(super) fn rulers_at_value(
         let scale = plot.transform.dvalue_dpos();
         let x_decimals = ((-scale[0].abs().log10()).ceil().at_least(0.0) as usize).at_most(6);
         let y_decimals = ((-scale[1].abs().log10()).ceil().at_least(0.0) as usize).at_most(6);
-        if plot.show_x && plot.show_y {
+        if let Some(custom_label) = custom_label_func {
+            custom_label(name, &value)
+        } else if plot.show_x && plot.show_y {
             format!(
                 "{}x = {:.*}\ny = {:.*}",
                 prefix, x_decimals, value.x, y_decimals, value.y

egui/src/widgets/plot/mod.rs

@@ -18,6 +18,8 @@ mod items;
 mod legend;
 mod transform;
 
+type CustomLabelFunc = Option<Box<dyn Fn(&str, &Value) -> String>>;
+
 // ----------------------------------------------------------------------------
 
 /// Information about the plot that has to persist between frames.
@@ -76,6 +78,7 @@ pub struct Plot {
 
     show_x: bool,
     show_y: bool,
+    custom_label_func: CustomLabelFunc,
     legend_config: Option<Legend>,
     show_background: bool,
     show_axes: [bool; 2],
@@ -102,6 +105,7 @@ impl Plot {
 
             show_x: true,
             show_y: true,
+            custom_label_func: None,
             legend_config: None,
             show_background: true,
             show_axes: [true; 2],
@@ -182,6 +186,32 @@ impl Plot {
         self
     }
 
+    /// Provide a function to customize the on-hovel label for the x and y axis
+    ///
+    /// ```
+    /// # egui::__run_test_ui(|ui| {
+    /// use egui::plot::{Line, Plot, Value, Values};
+    /// let sin = (0..1000).map(|i| {
+    ///     let x = i as f64 * 0.01;
+    ///     Value::new(x, x.sin())
+    /// });
+    /// let line = Line::new(Values::from_values_iter(sin));
+    /// Plot::new("my_plot").view_aspect(2.0)
+    /// .custom_label_func(Some(Box::new(|name, value| {
+    ///     if !name.is_empty() {
+    ///         format!("{}: {:.*}%", name, 1, value.y).to_string()
+    ///     } else {
+    ///         "".to_string()
+    ///     }
+    /// })))
+    /// .show(ui, |plot_ui| plot_ui.line(line));
+    /// # });
+    /// ```
+    pub fn custom_label_func(mut self, custom_lebel_func: CustomLabelFunc) -> Self {
+        self.custom_label_func = custom_lebel_func;
+        self
+    }
+
     /// Expand bounds to include the given x value.
     /// For instance, to always show the y axis, call `plot.include_x(0.0)`.
     pub fn include_x(mut self, x: impl Into<f64>) -> Self {
@@ -235,6 +265,7 @@ impl Plot {
             view_aspect,
             mut show_x,
             mut show_y,
+            custom_label_func,
             legend_config,
             show_background,
             show_axes,
@@ -406,6 +437,7 @@ impl Plot {
             items,
             show_x,
             show_y,
+            custom_label_func,
             show_axes,
             transform: transform.clone(),
         };
@@ -613,6 +645,7 @@ struct PreparedPlot {
     items: Vec<Box<dyn PlotItem>>,
     show_x: bool,
     show_y: bool,
+    custom_label_func: CustomLabelFunc,
     show_axes: [bool; 2],
     transform: ScreenTransform,
 }
@@ -731,6 +764,7 @@ impl PreparedPlot {
             transform,
             show_x,
             show_y,
+            custom_label_func,
             items,
             ..
         } = self;
@@ -760,10 +794,10 @@ impl PreparedPlot {
         };
 
         if let Some((item, elem)) = closest {
-            item.on_hover(elem, shapes, &plot);
+            item.on_hover(elem, shapes, &plot, custom_label_func);
         } else {
             let value = transform.value_from_position(pointer);
-            items::rulers_at_value(pointer, value, "", &plot, shapes);
+            items::rulers_at_value(pointer, value, "", &plot, shapes, custom_label_func);
         }
     }
 }