Improve documentation, simplify reference comparison

src/axis.rs

@@ -2,17 +2,29 @@
 /// except in the case where the value is outside of the axis range
 pub struct NormalisedValue(pub f32);
 
-/// A trait for creating an axis scale. It has two functions:
-/// `ticks` returns a vector of axis ticks
-/// `normalise` normalises the input `value`
+/// Specifies a generic scale on which axes and data can be rendered
 pub trait AxisScale {
+    /// Provides the list of [ticks](AxisTick) that should be rendered along the axis
     fn ticks(&self) -> Vec<AxisTick>;
+
+    /// Normalises a value within the axis scale to a number between 0 and 1,
+    /// where 0 represents the minimum value of the scale, and 1 the maximum
+    /// 
+    /// For example, for a linear scale between 50 and 100:
+    /// - normalise(50)  -> 0
+    /// - normalise(60)  -> 0.2
+    /// - normalise(75)  -> 0.5
+    /// - normalise(100) -> 1
     fn normalise(&self, value: f32) -> NormalisedValue;
 }
 
-/// An axis tick
-/// Each tick must have a location and a label.
+/// An axis tick, specifying a label to be displayed at some normalised 
+/// position along the axis
 pub struct AxisTick {
+    /// normalised location between zero and one along the axis specifying 
+    /// the position at which the tick should be rendered
     pub location: NormalisedValue,
+
+    /// text label that should be rendered alongside the tick
     pub label: String,
 }

src/horizontal_axis.rs

@@ -51,8 +51,10 @@ impl PartialEq for Props {
             && self.tick_len == other.tick_len
             && self.title == other.title
             && std::ptr::eq(
-                &*self.scale as *const dyn AxisScale as *const u8,
-                &*other.scale as *const dyn AxisScale as *const u8,
+                // test reference equality, avoiding issues with vtables discussed in
+                // https://github.com/rust-lang/rust/issues/46139
+                &*self.scale as *const _ as *const u8,
+                &*other.scale as *const _ as *const u8,
             )
     }
 }

src/horizontal_series.rs

@@ -74,13 +74,15 @@ impl PartialEq for Props {
             && self.y == other.y
             && self.height == other.height
             && self.width == other.width
+            // test reference equality, avoiding issues with vtables discussed in
+            // https://github.com/rust-lang/rust/issues/46139
             && std::ptr::eq(
-                &*self.horizontal_scale as *const dyn AxisScale as *const u8,
-                &*other.horizontal_scale as *const dyn AxisScale as *const u8,
+                &*self.horizontal_scale as *const _ as *const u8,
+                &*other.horizontal_scale as *const _ as *const u8,
             )
             && std::ptr::eq(
-                &*self.vertical_scale as *const dyn AxisScale as *const u8,
-                &*other.vertical_scale as *const dyn AxisScale as *const u8,
+                &*self.vertical_scale as *const _ as *const u8,
+                &*other.vertical_scale as *const _ as *const u8,
             )
     }
 }

src/vertical_axis.rs

@@ -51,8 +51,10 @@ impl PartialEq for Props {
             && self.tick_len == other.tick_len
             && self.title == other.title
             && std::ptr::eq(
-                &*self.scale as *const dyn AxisScale as *const u8,
-                &*other.scale as *const dyn AxisScale as *const u8,
+                // test reference equality, avoiding issues with vtables discussed in
+                // https://github.com/rust-lang/rust/issues/46139
+                &*self.scale as *const _ as *const u8,
+                &*other.scale as *const _ as *const u8,
             )
     }
 }