From 91029f39fc059f0cde65ba312e9fb9357a256f2e Mon Sep 17 00:00:00 2001
From: devteamaegis <devteam.aegis@gmail.com>
Date: Wed, 17 Jun 2026 15:12:14 -0400
Subject: [PATCH] fix: correct symlog scale invert to be inverse of apply

The symlog scaleTransform's JS invert computed sign(x)*exp(|x|-C),
subtracting the constant inside the exponent. The correct inverse of
apply (sign(x)*log1p(|x|)) is sign(x)*(exp(|x|)-C), matching the
existing sqlInvert. Previously invert(apply(5)) returned ~2.207
instead of 5, corrupting data coordinates when inverting symlog-scaled
pixel positions (e.g. plot brushing/zoom).
---
 packages/mosaic/sql/src/transforms/scales.ts |  2 +-
 packages/mosaic/sql/test/scales.test.ts      | 11 +++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)
 create mode 100644 packages/mosaic/sql/test/scales.test.ts

diff --git a/packages/mosaic/sql/src/transforms/scales.ts b/packages/mosaic/sql/src/transforms/scales.ts
index 49b9ae9d4..eeed09a71 100644
--- a/packages/mosaic/sql/src/transforms/scales.ts
+++ b/packages/mosaic/sql/src/transforms/scales.ts
@@ -88,7 +88,7 @@ function scaleSymlog({ constant = 1 } = {}): ScaleTransform<number> {
   const _ = +constant;
   return {
     apply: x => Math.sign(x) * Math.log1p(Math.abs(x)),
-    invert: x => Math.sign(x) * Math.exp(Math.abs(x) - _),
+    invert: x => Math.sign(x) * (Math.exp(Math.abs(x)) - _),
     sqlApply: c => (c = asNode(c), mul(sign(c), ln(add(_, abs(c))))),
     sqlInvert: c => mul(sign(c), sub(exp(abs(c)), _))
   };
diff --git a/packages/mosaic/sql/test/scales.test.ts b/packages/mosaic/sql/test/scales.test.ts
new file mode 100644
index 000000000..54c61ffdb
--- /dev/null
+++ b/packages/mosaic/sql/test/scales.test.ts
@@ -0,0 +1,11 @@
+import { expect, describe, it } from 'vitest';
+import { scaleTransform } from '../src/index.js';
+
+describe('scaleTransform', () => {
+  it('symlog invert is the inverse of apply', () => {
+    const s = scaleTransform<number>({ type: 'symlog' });
+    for (const x of [5, 10, 100, -7, 1, 0.5]) {
+      expect(s.invert(s.apply(x))).toBeCloseTo(x, 9);
+    }
+  });
+});