From 0f28ac96ed63a985bfc78d2072382712f788a4a8 Mon Sep 17 00:00:00 2001
From: Brandon Thomas <echelon@gmail.com>
Date: Wed, 14 Oct 2026 13:49:35 -0400
Subject: [PATCH] Fix up merged PRs: reversed category axes and trendline
 forecasts agree

#640 orders category slots bottom-to-top on horizontal bars (or reversed via
c:orientation) while #603 pads the axis for trendline forecasts. Slots now
include the forecast padding on the right side, and trendlines map their
category positions through the same slot order as the bars.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 crates/render/src/chart.rs | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/crates/render/src/chart.rs b/crates/render/src/chart.rs
index fe8df06a..4622e9c6 100644
--- a/crates/render/src/chart.rs
+++ b/crates/render/src/chart.rs
@@ -650,7 +650,9 @@ fn draw_inner(ctx: &mut RenderContext, rctx: &Ctx, c: &Chart, m: Affine, w: f64,
             }
         }
     }
-    draw_trendlines(ctx, rctx, c, m, plot, band, back, ncat, horiz, base, text_col, &to_axis);
+    // A category position (0-based, fractional) as a slot, the way `slot` places the bars.
+    let slot_at = |x: f64| if backwards { ncat as f64 - 1.0 - x + fwd } else { x + back };
+    draw_trendlines(ctx, rctx, c, m, plot, band, &slot_at, ncat, horiz, base, text_col, &to_axis);
     // Axis line.
     let mut ax = BezPath::new();
     if horiz {
@@ -744,7 +746,7 @@ const TREND_INK: [Rgba; 4] = [Rgba::rgb(0x22, 0x22, 0x22), Rgba::rgb(0xB0, 0x30,
 fn map_xy(
     plot: Rect,
     band: f64,
-    back: f64,
+    slot: &impl Fn(f64) -> f64,
     horiz: bool,
     scatter: bool,
     sx0: f64,
@@ -765,7 +767,7 @@ fn map_xy(
         plot.x0 + (x - sx0) / (sx1 - sx0) * plot.width()
     } else {
         let origin = if horiz { plot.y0 } else { plot.x0 };
-        origin + band * (x - 1.0 + back + 0.5)
+        origin + band * (slot(x - 1.0) + 0.5)
     };
     if !along.is_finite() {
         return None;
@@ -811,7 +813,7 @@ fn draw_trendlines(
     m: Affine,
     plot: Rect,
     band: f64,
-    back: f64,
+    slot: &impl Fn(f64) -> f64,
     ncat: usize,
     horiz: bool,
     base: f64,
@@ -861,8 +863,8 @@ fn draw_trendlines(
             let mut any = false;
             for w in samples.windows(2) {
                 let (Some(a), Some(b)) = (
-                    map_xy(plot, band, back, horiz, scatter, sx0, sx1, sec, w[0].0, w[0].1, to_axis),
-                    map_xy(plot, band, back, horiz, scatter, sx0, sx1, sec, w[1].0, w[1].1, to_axis),
+                    map_xy(plot, band, slot, horiz, scatter, sx0, sx1, sec, w[0].0, w[0].1, to_axis),
+                    map_xy(plot, band, slot, horiz, scatter, sx0, sx1, sec, w[1].0, w[1].1, to_axis),
                 ) else {
                     prev = None;
                     continue;