Allow cursors to be linked for plots (#1722)

* Allow cursors to be linked

* Link cursors in demo

* Refactor cursor memory to deal with removal of plots

* Refactor PlotItem::on_hover to produce a list of cursors

* Use a separate `LinkedCursorsGroup` type to link cursors.

* Refactor `Cursor`.

* Inline `push_argument_ruler` and `push_value_ruler`.

* Update documentation

* Update doc reference

crates/egui/src/widgets/plot/items/bar.rs

@@ -2,7 +2,7 @@ use crate::emath::NumExt;
 use crate::epaint::{Color32, RectShape, Rounding, Shape, Stroke};
 
 use super::{add_rulers_and_text, highlighted_color, Orientation, PlotConfig, RectElement};
-use crate::plot::{BarChart, PlotPoint, ScreenTransform};
+use crate::plot::{BarChart, Cursor, PlotPoint, ScreenTransform};
 
 /// One bar in a [`BarChart`]. Potentially floating, allowing stacked bar charts.
 /// Width can be changed to allow variable-width histograms.
@@ -142,13 +142,14 @@ impl Bar {
         parent: &BarChart,
         plot: &PlotConfig<'_>,
         shapes: &mut Vec<Shape>,
+        cursors: &mut Vec<Cursor>,
     ) {
         let text: Option<String> = parent
             .element_formatter
             .as_ref()
             .map(|fmt| fmt(self, parent));
 
-        add_rulers_and_text(self, plot, text, shapes);
+        add_rulers_and_text(self, plot, text, shapes, cursors);
     }
 }
 

crates/egui/src/widgets/plot/items/box_elem.rs

@@ -2,7 +2,7 @@ use crate::emath::NumExt;
 use crate::epaint::{Color32, RectShape, Rounding, Shape, Stroke};
 
 use super::{add_rulers_and_text, highlighted_color, Orientation, PlotConfig, RectElement};
-use crate::plot::{BoxPlot, PlotPoint, ScreenTransform};
+use crate::plot::{BoxPlot, Cursor, PlotPoint, ScreenTransform};
 
 /// Contains the values of a single box in a box plot.
 #[derive(Clone, Debug, PartialEq)]
@@ -221,13 +221,14 @@ impl BoxElem {
         parent: &BoxPlot,
         plot: &PlotConfig<'_>,
         shapes: &mut Vec<Shape>,
+        cursors: &mut Vec<Cursor>,
     ) {
         let text: Option<String> = parent
             .element_formatter
             .as_ref()
             .map(|fmt| fmt(self, parent));
 
-        add_rulers_and_text(self, plot, text, shapes);
+        add_rulers_and_text(self, plot, text, shapes, cursors);
     }
 }
 

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

@@ -7,7 +7,7 @@ use epaint::Mesh;
 
 use crate::*;
 
-use super::{LabelFormatter, PlotBounds, ScreenTransform};
+use super::{Cursor, LabelFormatter, PlotBounds, ScreenTransform};
 use rect_elem::*;
 use values::{ClosestElem, PlotGeometry};
 
@@ -72,6 +72,7 @@ pub(super) trait PlotItem {
         &self,
         elem: ClosestElem,
         shapes: &mut Vec<Shape>,
+        cursors: &mut Vec<Cursor>,
         plot: &PlotConfig<'_>,
         label_formatter: &LabelFormatter,
     ) {
@@ -96,7 +97,15 @@ pub(super) trait PlotItem {
         let pointer = plot.transform.position_from_point(&value);
         shapes.push(Shape::circle_filled(pointer, 3.0, line_color));
 
-        rulers_at_value(pointer, value, self.name(), plot, shapes, label_formatter);
+        rulers_at_value(
+            pointer,
+            value,
+            self.name(),
+            plot,
+            shapes,
+            cursors,
+            label_formatter,
+        );
     }
 }
 
@@ -1392,13 +1401,14 @@ impl PlotItem for BarChart {
         &self,
         elem: ClosestElem,
         shapes: &mut Vec<Shape>,
+        cursors: &mut Vec<Cursor>,
         plot: &PlotConfig<'_>,
         _: &LabelFormatter,
     ) {
         let bar = &self.bars[elem.index];
 
         bar.add_shapes(plot.transform, true, shapes);
-        bar.add_rulers_and_text(self, plot, shapes);
+        bar.add_rulers_and_text(self, plot, shapes, cursors);
     }
 }
 
@@ -1534,28 +1544,33 @@ impl PlotItem for BoxPlot {
         &self,
         elem: ClosestElem,
         shapes: &mut Vec<Shape>,
+        cursors: &mut Vec<Cursor>,
         plot: &PlotConfig<'_>,
         _: &LabelFormatter,
     ) {
         let box_plot = &self.boxes[elem.index];
 
         box_plot.add_shapes(plot.transform, true, shapes);
-        box_plot.add_rulers_and_text(self, plot, shapes);
+        box_plot.add_rulers_and_text(self, plot, shapes, cursors);
     }
 }
 
 // ----------------------------------------------------------------------------
 // Helper functions
 
-fn rulers_color(ui: &Ui) -> Color32 {
+pub(crate) fn rulers_color(ui: &Ui) -> Color32 {
     if ui.visuals().dark_mode {
         Color32::from_gray(100).additive()
     } else {
         Color32::from_black_alpha(180)
     }
 }
 
-fn vertical_line(pointer: Pos2, transform: &ScreenTransform, line_color: Color32) -> Shape {
+pub(crate) fn vertical_line(
+    pointer: Pos2,
+    transform: &ScreenTransform,
+    line_color: Color32,
+) -> Shape {
     let frame = transform.frame();
     Shape::line_segment(
         [
@@ -1566,7 +1581,11 @@ fn vertical_line(pointer: Pos2, transform: &ScreenTransform, line_color: Color32
     )
 }
 
-fn horizontal_line(pointer: Pos2, transform: &ScreenTransform, line_color: Color32) -> Shape {
+pub(crate) fn horizontal_line(
+    pointer: Pos2,
+    transform: &ScreenTransform,
+    line_color: Color32,
+) -> Shape {
     let frame = transform.frame();
     Shape::line_segment(
         [
@@ -1582,44 +1601,31 @@ fn add_rulers_and_text(
     plot: &PlotConfig<'_>,
     text: Option<String>,
     shapes: &mut Vec<Shape>,
+    cursors: &mut Vec<Cursor>,
 ) {
     let orientation = elem.orientation();
     let show_argument = plot.show_x && orientation == Orientation::Vertical
         || plot.show_y && orientation == Orientation::Horizontal;
     let show_values = plot.show_y && orientation == Orientation::Vertical
         || plot.show_x && orientation == Orientation::Horizontal;
 
-    let line_color = rulers_color(plot.ui);
-
     // Rulers for argument (usually vertical)
     if show_argument {
-        let push_argument_ruler = |argument: PlotPoint, shapes: &mut Vec<Shape>| {
-            let position = plot.transform.position_from_point(&argument);
-            let line = match orientation {
-                Orientation::Horizontal => horizontal_line(position, plot.transform, line_color),
-                Orientation::Vertical => vertical_line(position, plot.transform, line_color),
-            };
-            shapes.push(line);
-        };
-
         for pos in elem.arguments_with_ruler() {
-            push_argument_ruler(pos, shapes);
+            cursors.push(match orientation {
+                Orientation::Horizontal => Cursor::Horizontal { y: pos.y },
+                Orientation::Vertical => Cursor::Vertical { x: pos.x },
+            });
         }
     }
 
     // Rulers for values (usually horizontal)
     if show_values {
-        let push_value_ruler = |value: PlotPoint, shapes: &mut Vec<Shape>| {
-            let position = plot.transform.position_from_point(&value);
-            let line = match orientation {
-                Orientation::Horizontal => vertical_line(position, plot.transform, line_color),
-                Orientation::Vertical => horizontal_line(position, plot.transform, line_color),
-            };
-            shapes.push(line);
-        };
-
         for pos in elem.values_with_ruler() {
-            push_value_ruler(pos, shapes);
+            cursors.push(match orientation {
+                Orientation::Horizontal => Cursor::Vertical { x: pos.x },
+                Orientation::Vertical => Cursor::Horizontal { y: pos.y },
+            });
         }
     }
 
@@ -1656,14 +1662,14 @@ pub(super) fn rulers_at_value(
     name: &str,
     plot: &PlotConfig<'_>,
     shapes: &mut Vec<Shape>,
+    cursors: &mut Vec<Cursor>,
     label_formatter: &LabelFormatter,
 ) {
-    let line_color = rulers_color(plot.ui);
     if plot.show_x {
-        shapes.push(vertical_line(pointer, plot.transform, line_color));
+        cursors.push(Cursor::Vertical { x: value.x });
     }
     if plot.show_y {
-        shapes.push(horizontal_line(pointer, plot.transform, line_color));
+        cursors.push(Cursor::Horizontal { y: value.y });
     }
 
     let mut prefix = String::new();