Make sure plot size is positive (#4429)

* Closes #4425 

Fix: in Plot, Minimum values for screen protection.

Author: rustbasic

crates/egui_demo_lib/src/demo/context_menu.rs

@@ -97,11 +97,13 @@ impl super::View for ContextMenus {
                 egui::Grid::new("button_grid").show(ui, |ui| {
                     ui.add(
                         egui::DragValue::new(&mut self.width)
+                            .clamp_range(0.0..=f32::INFINITY)
                             .speed(1.0)
                             .prefix("Width: "),
                     );
                     ui.add(
                         egui::DragValue::new(&mut self.height)
+                            .clamp_range(0.0..=f32::INFINITY)
                             .speed(1.0)
                             .prefix("Height: "),
                     );

crates/egui_plot/src/lib.rs

@@ -742,7 +742,7 @@ impl Plot {
             margin_fraction,
             width,
             height,
-            min_size,
+            mut min_size,
             data_aspect,
             view_aspect,
             mut show_x,
@@ -773,6 +773,10 @@ impl Plot {
 
         // Determine position of widget.
         let pos = ui.available_rect_before_wrap().min;
+        // Minimum values for screen protection
+        min_size.x = min_size.x.at_least(1.0);
+        min_size.y = min_size.y.at_least(1.0);
+
         // Determine size of widget.
         let size = {
             let width = width