Formatter for plot axis labels (#1130)

* Fix Orientation not exposed, although there are public fields with its type

* Implement formatters for X/Y axis labels

* Use array instead of separate X/Y formatters

* Swap axis formatters if charts are horizontal

* Review suggestions

CHANGELOG.md

@@ -16,6 +16,7 @@ NOTE: [`epaint`](epaint/CHANGELOG.md), [`eframe`](eframe/CHANGELOG.md), [`egui_w
 * `Context::load_texture` to convert an image into a texture which can be displayed using e.g. `ui.image(texture, size)` ([#1110](https://github.com/emilk/egui/pull/1110)).
 * Added `Ui::add_visible` and `Ui::add_visible_ui`.
 * Added `CollapsingHeader::icon` to override the default open/close icon using a custom function. ([1147](https://github.com/emilk/egui/pull/1147))
+* Added `Plot::x_axis_formatter` and `Plot::y_axis_formatter` for custom axis labels ([#1130](https://github.com/emilk/egui/pull/1130))
 
 ### Changed 🔧
 * ⚠️ `Context::input` and `Ui::input` now locks a mutex. This can lead to a dead-lock is used in an `if let` binding!
@@ -36,6 +37,7 @@ NOTE: [`epaint`](epaint/CHANGELOG.md), [`eframe`](eframe/CHANGELOG.md), [`egui_w
 
 ### Fixed 🐛
 * Context menu now respects the theme ([#1043](https://github.com/emilk/egui/pull/1043))
+* Plot `Orientation` was not public, although fields using this type were ([#1130](https://github.com/emilk/egui/pull/1130))
 
 ### Contributors 🙏
 * [danielkeller](https://github.com/danielkeller): [#1050](https://github.com/emilk/egui/pull/1050).

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

@@ -9,11 +9,11 @@ use crate::*;
 
 use super::{CustomLabelFuncRef, PlotBounds, ScreenTransform};
 use rect_elem::*;
-use values::*;
+use values::{ClosestElem, PlotGeometry};
 
 pub use bar::Bar;
 pub use box_elem::{BoxElem, BoxSpread};
-pub use values::{LineStyle, MarkerShape, Value, Values};
+pub use values::{LineStyle, MarkerShape, Orientation, Value, Values};
 
 mod bar;
 mod box_elem;

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

@@ -125,6 +125,7 @@ impl ToString for LineStyle {
 
 // ----------------------------------------------------------------------------
 
+/// Determines whether a plot element is vertically or horizontally oriented.
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum Orientation {
     Horizontal,

egui/src/widgets/plot/mod.rs

@@ -10,7 +10,7 @@ use transform::{PlotBounds, ScreenTransform};
 
 pub use items::{
     Arrows, Bar, BarChart, BoxElem, BoxPlot, BoxSpread, HLine, Line, LineStyle, MarkerShape,
-    PlotImage, Points, Polygon, Text, VLine, Value, Values,
+    Orientation, PlotImage, Points, Polygon, Text, VLine, Value, Values,
 };
 pub use legend::{Corner, Legend};
 
@@ -21,6 +21,9 @@ mod transform;
 type CustomLabelFunc = dyn Fn(&str, &Value) -> String;
 type CustomLabelFuncRef = Option<Box<CustomLabelFunc>>;
 
+type AxisFormatterFn = dyn Fn(f64) -> String;
+type AxisFormatter = Option<Box<AxisFormatterFn>>;
+
 // ----------------------------------------------------------------------------
 
 /// Information about the plot that has to persist between frames.
@@ -80,6 +83,7 @@ pub struct Plot {
     show_x: bool,
     show_y: bool,
     custom_label_func: CustomLabelFuncRef,
+    axis_formatters: [AxisFormatter; 2],
     legend_config: Option<Legend>,
     show_background: bool,
     show_axes: [bool; 2],
@@ -107,6 +111,7 @@ impl Plot {
             show_x: true,
             show_y: true,
             custom_label_func: None,
+            axis_formatters: [None, None], // [None; 2] requires Copy
             legend_config: None,
             show_background: true,
             show_axes: [true; 2],
@@ -208,11 +213,35 @@ impl Plot {
     /// .show(ui, |plot_ui| plot_ui.line(line));
     /// # });
     /// ```
-    pub fn custom_label_func<F: 'static + Fn(&str, &Value) -> String>(
+    pub fn custom_label_func(
         mut self,
-        custom_lebel_func: F,
+        custom_label_func: impl Fn(&str, &Value) -> String + 'static,
     ) -> Self {
-        self.custom_label_func = Some(Box::new(custom_lebel_func));
+        self.custom_label_func = Some(Box::new(custom_label_func));
+        self
+    }
+
+    /// Provide a function to customize the labels for the X axis.
+    ///
+    /// This is useful for custom input domains, e.g. date/time.
+    ///
+    /// If axis labels should not appear for certain values or beyond a certain zoom/resolution,
+    /// the formatter function can return empty strings. This is also useful if your domain is
+    /// discrete (e.g. only full days in a calendar).
+    pub fn x_axis_formatter(mut self, func: impl Fn(f64) -> String + 'static) -> Self {
+        self.axis_formatters[0] = Some(Box::new(func));
+        self
+    }
+
+    /// Provide a function to customize the labels for the Y axis.
+    ///
+    /// This is useful for custom value representation, e.g. percentage or units.
+    ///
+    /// If axis labels should not appear for certain values or beyond a certain zoom/resolution,
+    /// the formatter function can return empty strings. This is also useful if your Y values are
+    /// discrete (e.g. only integers).
+    pub fn y_axis_formatter(mut self, func: impl Fn(f64) -> String + 'static) -> Self {
+        self.axis_formatters[1] = Some(Box::new(func));
         self
     }
 
@@ -270,6 +299,7 @@ impl Plot {
             mut show_x,
             mut show_y,
             custom_label_func,
+            axis_formatters,
             legend_config,
             show_background,
             show_axes,
@@ -442,6 +472,7 @@ impl Plot {
             show_x,
             show_y,
             custom_label_func,
+            axis_formatters,
             show_axes,
             transform: transform.clone(),
         };
@@ -650,6 +681,7 @@ struct PreparedPlot {
     show_x: bool,
     show_y: bool,
     custom_label_func: CustomLabelFuncRef,
+    axis_formatters: [AxisFormatter; 2],
     show_axes: [bool; 2],
     transform: ScreenTransform,
 }
@@ -680,7 +712,11 @@ impl PreparedPlot {
     }
 
     fn paint_axis(&self, ui: &Ui, axis: usize, shapes: &mut Vec<Shape>) {
-        let Self { transform, .. } = self;
+        let Self {
+            transform,
+            axis_formatters,
+            ..
+        } = self;
 
         let bounds = transform.bounds();
 
@@ -740,8 +776,15 @@ impl PreparedPlot {
 
             if text_alpha > 0.0 {
                 let color = color_from_alpha(ui, text_alpha);
-                let text = emath::round_to_decimals(value_main, 5).to_string(); // hack
 
+                let text: String = if let Some(formatter) = axis_formatters[axis].as_deref() {
+                    formatter(value_main)
+                } else {
+                    emath::round_to_decimals(value_main, 5).to_string() // hack
+                };
+
+                // Custom formatters can return empty string to signal "no label at this resolution"
+                if !text.is_empty() {
                     let galley = ui.painter().layout_no_wrap(text, font_id.clone(), color);
 
                     let mut text_pos = pos_in_gui + vec2(1.0, -galley.size().y);
@@ -754,6 +797,7 @@ impl PreparedPlot {
                     shapes.push(Shape::galley(text_pos, galley));
                 }
             }
+        }
 
         fn color_from_alpha(ui: &Ui, alpha: f32) -> Color32 {
             if ui.visuals().dark_mode {

egui_demo_lib/src/apps/demo/plot_demo.rs

@@ -71,8 +71,8 @@ impl LineDemo {
 
             ui.vertical(|ui| {
                 ui.style_mut().wrap = Some(false);
-                ui.checkbox(animate, "animate");
+                ui.checkbox(animate, "Animate");
-                ui.checkbox(square, "square view")
+                ui.checkbox(square, "Square view")
                     .on_hover_text("Always keep the viewport square.");
                 ui.checkbox(proportional, "Proportional data axes")
                     .on_hover_text("Tick are the same size on both axes.");
@@ -523,15 +523,40 @@ impl ChartsDemo {
         .name("Set 4")
         .stack_on(&[&chart1, &chart2, &chart3]);
 
+        let mut x_fmt: fn(f64) -> String = |val| {
+            if val >= 0.0 && val <= 4.0 && is_approx_integer(val) {
+                // Only label full days from 0 to 4
+                format!("Day {}", val)
+            } else {
+                // Otherwise return empty string (i.e. no label)
+                String::new()
+            }
+        };
+
+        let mut y_fmt: fn(f64) -> String = |val| {
+            let percent = 100.0 * val;
+
+            if is_approx_integer(percent) && !is_approx_zero(percent) {
+                // Only show integer percentages,
+                // and don't show at Y=0 (label overlaps with X axis label)
+                format!("{}%", percent)
+            } else {
+                String::new()
+            }
+        };
+
         if !self.vertical {
             chart1 = chart1.horizontal();
             chart2 = chart2.horizontal();
             chart3 = chart3.horizontal();
             chart4 = chart4.horizontal();
+            std::mem::swap(&mut x_fmt, &mut y_fmt);
         }
 
         Plot::new("Stacked Bar Chart Demo")
             .legend(Legend::default())
+            .x_axis_formatter(x_fmt)
+            .y_axis_formatter(y_fmt)
             .data_aspect(1.0)
             .show(ui, |plot_ui| {
                 plot_ui.bar_chart(chart1);
@@ -698,3 +723,11 @@ impl super::View for PlotDemo {
         }
     }
 }
+
+fn is_approx_zero(val: f64) -> bool {
+    val.abs() < 1e-6
+}
+
+fn is_approx_integer(val: f64) -> bool {
+    val.fract().abs() < 1e-6
+}