Expand plot axes thickness to fit their labels (#3921)

Expand the plot axis thickness as the contained plot axis labels get
wider.

This fixes a problem where the plot labels would otherwise get clipped.


![plot-axis-expansion](https://github.com/emilk/egui/assets/1148717/4500a26e-4a11-401d-9e8e-2d98d02ef3b7)

crates/egui_plot/src/memory.rs

@@ -1,3 +1,5 @@
+use std::collections::BTreeMap;
+
 use egui::{ahash, Context, Id, Pos2, Vec2b};
 
 use crate::{PlotBounds, PlotTransform};
@@ -23,6 +25,13 @@ pub struct PlotMemory {
 
     /// Allows to remember the first click position when performing a boxed zoom
     pub(crate) last_click_pos_for_zoom: Option<Pos2>,
+
+    /// The thickness of each of the axes the previous frame.
+    ///
+    /// This is used in the next frame to make the axes thicker
+    /// in order to fit the labels, if necessary.
+    pub(crate) x_axis_thickness: BTreeMap<usize, f32>,
+    pub(crate) y_axis_thickness: BTreeMap<usize, f32>,
 }
 
 impl PlotMemory {