From 53972cf37c2fd20a2383abb851a2a4ee73a6671a Mon Sep 17 00:00:00 2001
From: NetterSilver <133535268+Silvertree2010@users.noreply.github.com>
Date: Thu, 22 Oct 2026 21:07:43 +0000
Subject: [PATCH] Make chart axis flatness thresholds relative to data
 magnitude

Found while testing crates/model/src/chart.rs:775-794 (axis_scale); same pattern in nice_scale :817

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
---
 crates/model/src/chart.rs | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/crates/model/src/chart.rs b/crates/model/src/chart.rs
index f5aeb1ee..29d28fbb 100644
--- a/crates/model/src/chart.rs
+++ b/crates/model/src/chart.rs
@@ -772,7 +772,7 @@ pub fn axis_scale(
     // Office's automatic limits ("How chart axis limits are determined", as Jon Peltier
     // summarises it): 5 % of the data range past the data, and zero when the values don't
     // sit within the top sixth of their largest magnitude. A single value is measured from zero.
-    let flat = hi - lo < 1e-12;
+    let flat = hi - lo <= 1e-12 * lo.abs().max(hi.abs());
     let pad = if !headroom {
         0.0
     } else if flat {
@@ -784,12 +784,15 @@ pub fn axis_scale(
     let auto_hi = if hi <= 0.0 && (flat || hi > lo * 5.0 / 6.0) { 0.0 } else { hi + pad };
     let mut lo = fmin.unwrap_or(auto_lo);
     let mut hi = fmax.unwrap_or(auto_hi);
-    if hi - lo < 1e-12 {
+    let mag = lo.abs().max(hi.abs());
+    // Relative to the data's magnitude, so tiny values don't count as flat; all-zero falls back to 1.
+    let unit = if mag == 0.0 { 1.0 } else { mag };
+    if hi - lo <= 1e-12 * mag {
         // A fixed limit on the far side of the data: the automatic end gives way.
         match (fmin, fmax) {
-            (None, Some(_)) => lo = if hi > 0.0 { 0.0 } else { hi - hi.abs().max(1.0) },
+            (None, Some(_)) => lo = if hi > 0.0 { 0.0 } else { hi - hi.abs().max(unit) },
             (Some(_), None) if lo < 0.0 => hi = 0.0,
-            _ => hi = lo + lo.abs().max(1.0),
+            _ => hi = lo + unit,
         }
     }
     let n = max_intervals.clamp(1, 50) as f64;
@@ -814,7 +817,9 @@ pub fn axis_scale(
 /// "Nice" axis scale: (min, max, major step) covering [lo, hi].
 pub fn nice_scale(lo: f64, hi: f64) -> (f64, f64, f64) {
     let (lo, hi) = if lo.is_finite() && hi.is_finite() { (lo.min(hi), lo.max(hi)) } else { (0.0, 1.0) };
-    let span = if hi - lo < 1e-12 { hi.abs().max(1.0) } else { hi - lo };
+    let data_mag = lo.abs().max(hi.abs());
+    let span = if hi - lo <= 1e-12 * data_mag { data_mag } else { hi - lo };
+    let span = if span == 0.0 { 1.0 } else { span };
     let raw = span / 5.0;
     let mag = 10f64.powf(raw.log10().floor());
     let norm = raw / mag;
@@ -831,7 +836,7 @@ pub fn nice_scale(lo: f64, hi: f64) -> (f64, f64, f64) {
     } * mag;
     let min = (lo / step).floor() * step;
     let max = (hi / step).ceil() * step;
-    let max = if (max - min).abs() < 1e-12 { min + step } else { max };
+    let max = if (max - min).abs() <= 1e-12 * data_mag { min + step } else { max };
     (min, max, step)
 }
 
@@ -839,6 +844,16 @@ pub fn nice_scale(lo: f64, hi: f64) -> (f64, f64, f64) {
 mod tests {
     use super::*;
 
+    #[test]
+    fn tiny_data_keeps_its_own_scale() {
+        let (min, max, step) = axis_scale(1e-13, 5e-13, None, None, None, 8, true);
+        assert!(min == 0.0 && (5e-13..1e-11).contains(&max), "{min} {max} {step}");
+        assert!(step > 0.0 && step < 1e-11);
+        let (min, max, step) = nice_scale(1e-13, 5e-13);
+        assert!(min <= 1e-13 && (5e-13..1e-11).contains(&max) && step < 1e-11, "{min} {max} {step}");
+        assert_eq!(axis_scale(0.0, 0.0, None, None, None, 8, true).1, 1.0);
+    }
+
     #[test]
     fn log_axes_run_over_whole_powers() {
         let mut c = Chart::sample(ChartType::Line);