Implement custom ruler color for Plot (#47)

Implement custom ruler color for Plot which is exposed through the new
`custom_ruler_color` method. The entire thing is only a few lines.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

Author: gweisert

egui_plot/src/lib.rs

@@ -180,6 +180,7 @@ pub struct Plot<'a> {
     x_axes: Vec<AxisHints<'a>>, // default x axes
     y_axes: Vec<AxisHints<'a>>, // default y axes
     legend_config: Option<Legend>,
+    cursor_color: Option<Color32>,
     show_background: bool,
     show_axes: Vec2b,
 
@@ -227,6 +228,7 @@ impl<'a> Plot<'a> {
             x_axes: vec![AxisHints::new(Axis::X)],
             y_axes: vec![AxisHints::new(Axis::Y)],
             legend_config: None,
+            cursor_color: None,
             show_background: true,
             show_axes: true.into(),
 
@@ -713,6 +715,15 @@ impl<'a> Plot<'a> {
         self
     }
 
+    /// Set custom cursor color.
+    ///
+    /// You may set the color to [`Color32::TRANSPARENT`] to hide the cursors.
+    #[inline]
+    pub fn cursor_color(mut self, color: Color32) -> Self {
+        self.cursor_color = Some(color);
+        self
+    }
+
     /// Interact with and add items to the plot and finally draw it.
     pub fn show<R>(
         self,
@@ -754,6 +765,7 @@ impl<'a> Plot<'a> {
             x_axes,
             y_axes,
             legend_config,
+            cursor_color,
             reset,
             show_background,
             show_axes,
@@ -1182,6 +1194,7 @@ impl<'a> Plot<'a> {
             draw_cursor_x: linked_cursors.as_ref().map_or(false, |group| group.1.x),
             draw_cursor_y: linked_cursors.as_ref().map_or(false, |group| group.1.y),
             draw_cursors,
+            cursor_color,
             grid_spacers,
             sharp_grid_lines,
             clamp_grid,
@@ -1472,6 +1485,7 @@ struct PreparedPlot<'a> {
     draw_cursor_x: bool,
     draw_cursor_y: bool,
     draw_cursors: Vec<Cursor>,
+    cursor_color: Option<Color32>,
 
     sharp_grid_lines: bool,
     clamp_grid: bool,
@@ -1513,7 +1527,7 @@ impl<'a> PreparedPlot<'a> {
         };
 
         // Draw cursors
-        let line_color = rulers_color(ui);
+        let line_color = self.cursor_color.unwrap_or_else(|| rulers_color(ui));
 
         let mut draw_cursor = |cursors: &Vec<Cursor>, always| {
             for &cursor in cursors {