Spit crosshair and labels API (#221)

Now you can have one without the other. Addresses #217

# Public API changes

The PlotItem label formatter is now optional:
```
-pub fn egui_plot::PlotItem::on_hover(&self, plot_area_response: &egui::response::Response, elem: egui_plot::ClosestElem, shapes: &mut alloc::vec::Vec<epaint::shapes::shape::Shape>, cursors: &mut alloc::vec::Vec<egui_plot::Cursor>, plot: &egui_plot::PlotConfig<'_>, label_formatter: &egui_plot::LabelFormatter<'_>)
+pub fn egui_plot::PlotItem::on_hover(&self, plot_area_response: &egui::response::Response, elem: egui_plot::ClosestElem, shapes: &mut alloc::vec::Vec<epaint::shapes::shape::Shape>, cursors: &mut alloc::vec::Vec<egui_plot::Cursor>, plot: &egui_plot::PlotConfig<'_>, label_formatter: &core::option::Option<egui_plot::LabelFormatter<'_>>)
```

Allow to specify crosshair:
```
+pub fn egui_plot::Plot<'a>::show_crosshair(self, show: bool) -> Self
+pub egui_plot::PlotConfig::show_crosshair: bool
```

Add default formatter:
```
+pub fn egui_plot::default_label_formatter(name: &str, value: &egui_plot::PlotPoint) -> alloc::string::String
```

And by default, labels are not shown. You have to enable it by passing
`label_formatter(you_label_formatter)`
You can use the `default_label_formatter`.

Author: michalsustr

egui_plot/src/items/bar_chart.rs

@@ -210,7 +210,7 @@ impl PlotItem for BarChart {
         shapes: &mut Vec<Shape>,
         cursors: &mut Vec<Cursor>,
         plot: &PlotConfig<'_>,
-        _: &LabelFormatter<'_>,
+        _: &Option<LabelFormatter<'_>>,
     ) {
         let bar = &self.bars[elem.index];
 

egui_plot/src/items/box_plot.rs

@@ -176,7 +176,7 @@ impl PlotItem for BoxPlot {
         shapes: &mut Vec<Shape>,
         cursors: &mut Vec<Cursor>,
         plot: &PlotConfig<'_>,
-        _: &LabelFormatter<'_>,
+        _: &Option<LabelFormatter<'_>>,
     ) {
         let box_plot = &self.boxes[elem.index];
 

egui_plot/src/items/heatmap.rs

@@ -460,7 +460,7 @@ impl PlotItem for Heatmap {
         shapes: &mut Vec<Shape>,
         _cursors: &mut Vec<Cursor>,
         plot: &PlotConfig<'_>,
-        _: &LabelFormatter<'_>,
+        _: &Option<LabelFormatter<'_>>,
     ) {
         let (rect, color, text) = self.tile_view_info(plot.ui, plot.transform, elem.index);
         let mut mesh = Mesh::default();

egui_plot/src/items/mod.rs

@@ -9,7 +9,6 @@ use std::ops::RangeInclusive;
 use egui::Align2;
 use egui::Color32;
 use egui::Id;
-use egui::NumExt as _;
 use egui::PopupAnchor;
 use egui::Pos2;
 use egui::Shape;
@@ -92,6 +91,9 @@ pub struct PlotConfig<'a> {
 
     /// Whether to show the y-axis value.
     pub show_y: bool,
+
+    /// Whether to show the crosshair rulers.
+    pub show_crosshair: bool,
 }
 
 /// Trait shared by things that can be drawn in the plot.
@@ -172,7 +174,7 @@ pub trait PlotItem {
         shapes: &mut Vec<Shape>,
         cursors: &mut Vec<Cursor>,
         plot: &PlotConfig<'_>,
-        label_formatter: &LabelFormatter<'_>,
+        label_formatter: &Option<LabelFormatter<'_>>,
     ) {
         let points = match self.geometry() {
             PlotGeometry::Points(points) => points,
@@ -273,44 +275,28 @@ pub(super) fn rulers_and_tooltip_at_value(
     name: &str,
     plot: &PlotConfig<'_>,
     cursors: &mut Vec<Cursor>,
-    label_formatter: &LabelFormatter<'_>,
+    label_formatter: &Option<LabelFormatter<'_>>,
 ) {
-    if plot.show_x {
-        cursors.push(Cursor::Vertical { x: value.x });
-    }
-    if plot.show_y {
-        cursors.push(Cursor::Horizontal { y: value.y });
-    }
-
-    let text = if let Some(custom_label) = label_formatter {
-        let label = custom_label(name, &value);
-        if label.is_empty() {
-            return;
+    // Add crosshair rulers if enabled
+    if plot.show_crosshair {
+        if plot.show_x {
+            cursors.push(Cursor::Vertical { x: value.x });
         }
-        label
-    } else {
-        let prefix = if name.is_empty() {
-            String::new()
-        } else {
-            format!("{name}\n")
-        };
-        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 {
-            format!(
-                "{}x = {:.*}\ny = {:.*}",
-                prefix, x_decimals, value.x, y_decimals, value.y
-            )
-        } else if plot.show_x {
-            format!("{}x = {:.*}", prefix, x_decimals, value.x)
-        } else if plot.show_y {
-            format!("{}y = {:.*}", prefix, y_decimals, value.y)
-        } else {
-            unreachable!()
+        if plot.show_y {
+            cursors.push(Cursor::Horizontal { y: value.y });
         }
+    }
+
+    // Only show tooltip if label_formatter is provided
+    let Some(custom_label) = label_formatter else {
+        return;
     };
 
+    let text = custom_label(name, &value);
+    if text.is_empty() {
+        return;
+    }
+
     // 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(),

egui_plot/src/label.rs

@@ -19,4 +19,15 @@ pub fn format_number(number: f64, num_decimals: usize) -> String {
 type LabelFormatterFn<'a> = dyn Fn(&str, &PlotPoint) -> String + 'a;
 
 /// Optional label formatter function for customizing hover labels.
-pub type LabelFormatter<'a> = Option<Box<LabelFormatterFn<'a>>>;
+pub type LabelFormatter<'a> = Box<LabelFormatterFn<'a>>;
+
+/// 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)
+}

egui_plot/src/lib.rs

@@ -62,6 +62,7 @@ pub use crate::items::Span;
 pub use crate::items::Text;
 pub use crate::items::VLine;
 pub use crate::label::LabelFormatter;
+pub use crate::label::default_label_formatter;
 pub use crate::label::format_number;
 pub use crate::memory::PlotMemory;
 pub use crate::overlays::ColorConflictHandling;

egui_plot/src/plot.rs

@@ -115,7 +115,8 @@ pub struct Plot<'a> {
 
     show_x: bool,
     show_y: bool,
-    label_formatter: LabelFormatter<'a>,
+    show_crosshair: bool,
+    label_formatter: Option<LabelFormatter<'a>>,
     coordinates_formatter: Option<(Corner, CoordinatesFormatter<'a>)>,
     x_axes: Vec<AxisHints<'a>>, // default x axes
     y_axes: Vec<AxisHints<'a>>, // default y axes
@@ -166,6 +167,7 @@ impl<'a> Plot<'a> {
 
             show_x: true,
             show_y: true,
+            show_crosshair: true,
             label_formatter: None,
             coordinates_formatter: None,
             x_axes: vec![AxisHints::new(Axis::X)],
@@ -272,6 +274,13 @@ impl<'a> Plot<'a> {
         self
     }
 
+    /// Show the crosshair when hovering. Default: `true`.
+    #[inline]
+    pub fn show_crosshair(mut self, show: bool) -> Self {
+        self.show_crosshair = show;
+        self
+    }
+
     /// Always keep the X-axis centered. Default: `false`.
     #[inline]
     pub fn center_x_axis(mut self, on: bool) -> Self {
@@ -1543,6 +1552,7 @@ impl<'a> Plot<'a> {
             transform,
             show_x: show_xy.x,
             show_y: show_xy.y,
+            show_crosshair: self.show_crosshair,
         };
 
         let mut cursors = Vec::new();

examples/lines/src/app.rs

@@ -15,6 +15,7 @@ use egui_plot::Line;
 use egui_plot::LineStyle;
 use egui_plot::Plot;
 use egui_plot::PlotPoints;
+use egui_plot::default_label_formatter;
 
 #[derive(Clone, Copy, PartialEq)]
 pub struct LineExample {
@@ -27,6 +28,8 @@ pub struct LineExample {
     coordinates: bool,
     show_axes: bool,
     show_grid: bool,
+    show_crosshair: bool,
+    show_labels: bool,
     line_style: LineStyle,
     gradient: bool,
     gradient_fill: bool,
@@ -46,6 +49,8 @@ impl Default for LineExample {
             coordinates: true,
             show_axes: true,
             show_grid: true,
+            show_crosshair: true,
+            show_labels: true,
             line_style: LineStyle::Solid,
             gradient: false,
             gradient_fill: false,
@@ -61,31 +66,25 @@ impl LineExample {
             ui.group(|ui| {
                 ui.vertical(|ui| {
                     ui.label("Circle:");
-                    ui.add(
-                        egui::DragValue::new(&mut self.circle_radius)
-                            .speed(0.1)
-                            .range(0.0..=f64::INFINITY)
-                            .prefix("r: "),
-                    );
+                    ui.add(egui::DragValue::new(&mut self.circle_radius).speed(0.1).prefix("r: "));
                     ui.horizontal(|ui| {
                         ui.add(egui::DragValue::new(&mut self.circle_center.x).speed(0.1).prefix("x: "));
                         ui.add(egui::DragValue::new(&mut self.circle_center.y).speed(1.0).prefix("y: "));
                     });
-                })
+                });
             });
             ui.vertical(|ui| {
                 ui.checkbox(&mut self.show_axes, "Show axes");
                 ui.checkbox(&mut self.show_grid, "Show grid");
-                ui.checkbox(&mut self.coordinates, "Show coordinates on hover")
-                    .on_hover_text("Can take a custom formatting function.");
+                ui.checkbox(&mut self.show_crosshair, "Show crosshair");
+                ui.checkbox(&mut self.coordinates, "Show coordinates");
+                ui.checkbox(&mut self.show_labels, "Show hover labels");
             });
             ui.vertical(|ui| {
                 ui.style_mut().wrap_mode = Some(TextWrapMode::Extend);
                 ui.checkbox(&mut self.animate, "Animate");
-                ui.checkbox(&mut self.square, "Square view")
-                    .on_hover_text("Always keep the viewport square.");
-                ui.checkbox(&mut self.proportional, "Proportional data axes")
-                    .on_hover_text("Tick are the same size on both axes.");
+                ui.checkbox(&mut self.square, "Square view");
+                ui.checkbox(&mut self.proportional, "Proportional data axes");
                 ComboBox::from_label("Line style")
                     .selected_text(self.line_style.to_string())
                     .show_ui(ui, |ui| {
@@ -103,8 +102,6 @@ impl LineExample {
             ui.vertical(|ui| {
                 ui.checkbox(&mut self.gradient, "Gradient line");
                 ui.add_enabled(self.gradient, Checkbox::new(&mut self.gradient_fill, "Gradient fill"));
-            });
-            ui.vertical(|ui| {
                 ui.checkbox(&mut self.invert_x, "Invert X axis");
                 ui.checkbox(&mut self.invert_y, "Invert Y axis");
             });
@@ -113,10 +110,9 @@ impl LineExample {
     }
 
     fn circle(&self) -> Line<'_> {
-        let n = 512;
-        let points: PlotPoints<'_> = (0..=n)
+        let points: PlotPoints<'_> = (0..=512)
             .map(|i| {
-                let t = egui::remap(i as f64, 0.0..=(n as f64), 0.0..=TAU);
+                let t = egui::remap(i as f64, 0.0..=512.0, 0.0..=TAU);
                 [
                     self.circle_radius * t.cos() + self.circle_center.x as f64,
                     self.circle_radius * t.sin() + self.circle_center.y as f64,
@@ -166,11 +162,11 @@ impl LineExample {
             ui.ctx().request_repaint();
             self.time += ui.input(|i| i.unstable_dt).at_most(1.0 / 30.0) as f64;
         }
-
         let mut plot = Plot::new("lines_demo")
             .legend(Legend::default().title("Lines"))
             .show_axes(self.show_axes)
             .show_grid(self.show_grid)
+            .show_crosshair(self.show_crosshair)
             .invert_x(self.invert_x)
             .invert_y(self.invert_y);
         if self.square {
@@ -182,6 +178,9 @@ impl LineExample {
         if self.coordinates {
             plot = plot.coordinates_formatter(Corner::LeftBottom, CoordinatesFormatter::default());
         }
+        if self.show_labels {
+            plot = plot.label_formatter(default_label_formatter);
+        }
         plot.show(ui, |plot_ui| {
             plot_ui.line(self.circle());
             plot_ui.line(self.sin());