From 03f3d112a2ef8fd9f1eef29a4d822479440e80bf Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Thu, 2 Apr 2026 06:33:03 +0000
Subject: [PATCH 1/3] feat: support | 0 + var for row faceting in formula
 syntax

Agent-Logs-Url: https://github.com/yihui/gglite/sessions/3a6cd822-fa4d-4ac0-b226-eee4af9726f3

Co-authored-by: yihui-bot <264330240+yihui-bot@users.noreply.github.com>
---
 DESCRIPTION               |  2 +-
 NEWS.md                   |  3 +++
 R/utils.R                 | 13 +++++++++----
 tests/testit/test-utils.R |  9 +++++++++
 4 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/DESCRIPTION b/DESCRIPTION
index a9990fc..05d3a06 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,6 +1,6 @@
 Package: gglite
 Title: Lightweight Data Visualization via the Grammar of Graphics
-Version: 0.0.10
+Version: 0.0.11
 Authors@R: person("Yihui", "Xie", role = c("aut", "cre"), email = "xie@yihui.name",
     comment = c(ORCID = "0000-0003-0645-5666"))
 Description: A lightweight R interface to the AntV G2 JavaScript visualization
diff --git a/NEWS.md b/NEWS.md
index 0624123..e1a662c 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,8 @@
 # gglite 0.1
 
+- The formula syntax in `g2()` now supports `| 0 + var` for row faceting (e.g.,
+  `y ~ x | 0 + var`), in addition to the existing `| var` for column faceting.
+
 A lightweight R interface to the
 [AntV G2](https://g2.antv.antgroup.com/) JavaScript visualization library with
 a ggplot2-style API. Create interactive charts using the Grammar of Graphics
diff --git a/R/utils.R b/R/utils.R
index 14289f8..6db95e9 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -118,7 +118,8 @@ extract_terms = function(expr) {
 #' - `y ~ x` maps to `list(x = 'x', y = 'y')`
 #' - `~ x` maps to `list(x = 'x')`
 #' - `~ x1 + x2 + x3` maps to `list(position = c('x1', 'x2', 'x3'))`
-#' - `y ~ x | z` adds faceting by `z`
+#' - `y ~ x | z` adds faceting by `z` (columns)
+#' - `y ~ x | 0 + z` adds faceting by `z` (rows)
 #' - `y ~ x | z1 + z2` adds faceting by `z1` (columns) and `z2` (rows)
 #'
 #' @param f A formula object.
@@ -147,12 +148,16 @@ parse_formula = function(f) {
     aesthetics$position = rhs_terms
   }
 
-  # Build facet
+  # Build facet: | var → column; | 0 + var → row; | var1 + var2 → both
   facet = NULL
   if (length(facet_terms)) {
     enc = list()
-    if (length(facet_terms) >= 1) enc$x = facet_terms[1]
-    if (length(facet_terms) >= 2) enc$y = facet_terms[2]
+    row_only = facet_terms[1] == '0'
+    if (row_only) facet_terms = facet_terms[-1]
+    if (length(facet_terms) >= 1) {
+      if (row_only) enc$y = facet_terms[1] else enc$x = facet_terms[1]
+    }
+    if (!row_only && length(facet_terms) >= 2) enc$y = facet_terms[2]
     facet = list(type = 'facetRect', encode = enc)
   }
 
diff --git a/tests/testit/test-utils.R b/tests/testit/test-utils.R
index 55428e1..8987f01 100644
--- a/tests/testit/test-utils.R
+++ b/tests/testit/test-utils.R
@@ -68,6 +68,15 @@ assert('parse_formula: y ~ x | z1 + z2', {
   (res$facet$encode$y %==% 'b')
 })
 
+assert('parse_formula: y ~ x | 0 + z (row facet)', {
+  res = parse_formula(hp ~ mpg | 0 + cyl)
+  (res$aesthetics$x %==% 'mpg')
+  (res$aesthetics$y %==% 'hp')
+  (res$facet$type %==% 'facetRect')
+  (is.null(res$facet$encode$x))
+  (res$facet$encode$y %==% 'cyl')
+})
+
 assert('ts_to_df converts univariate ts', {
   res = ts_to_df(sunspot.year)
   (is.data.frame(res$data))

From 1af7a61caf2b2233a256593e29cdd59f7e8c21cc Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Thu, 2 Apr 2026 06:34:44 +0000
Subject: [PATCH 2/3] guard against bare | 0 edge case, add test

Agent-Logs-Url: https://github.com/yihui/gglite/sessions/3a6cd822-fa4d-4ac0-b226-eee4af9726f3

Co-authored-by: yihui-bot <264330240+yihui-bot@users.noreply.github.com>
---
 R/utils.R                 | 2 +-
 tests/testit/test-utils.R | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/R/utils.R b/R/utils.R
index 6db95e9..cedfb89 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -158,7 +158,7 @@ parse_formula = function(f) {
       if (row_only) enc$y = facet_terms[1] else enc$x = facet_terms[1]
     }
     if (!row_only && length(facet_terms) >= 2) enc$y = facet_terms[2]
-    facet = list(type = 'facetRect', encode = enc)
+    if (length(enc)) facet = list(type = 'facetRect', encode = enc)
   }
 
   list(aesthetics = aesthetics, facet = facet)
diff --git a/tests/testit/test-utils.R b/tests/testit/test-utils.R
index 8987f01..22b2b69 100644
--- a/tests/testit/test-utils.R
+++ b/tests/testit/test-utils.R
@@ -77,6 +77,11 @@ assert('parse_formula: y ~ x | 0 + z (row facet)', {
   (res$facet$encode$y %==% 'cyl')
 })
 
+assert('parse_formula: y ~ x | 0 (bare 0, no facet)', {
+  res = parse_formula(hp ~ mpg | 0)
+  (is.null(res$facet))
+})
+
 assert('ts_to_df converts univariate ts', {
   res = ts_to_df(sunspot.year)
   (is.data.frame(res$data))

From 807ffb1376c62d192bf61f682073667de239ed3a Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Thu, 2 Apr 2026 06:57:20 +0000
Subject: [PATCH 3/3] address PR feedback: revert NEWS.md, compact facet logic,
 add vignette examples

Agent-Logs-Url: https://github.com/yihui/gglite/sessions/a4f72763-bd38-4357-afa0-7214c0c67f75

Co-authored-by: yihui-bot <264330240+yihui-bot@users.noreply.github.com>
---
 NEWS.md              |  3 ---
 R/utils.R            | 11 ++++-------
 vignettes/gglite.Rmd | 18 ++++++++++++++++++
 3 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index e1a662c..0624123 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,8 +1,5 @@
 # gglite 0.1
 
-- The formula syntax in `g2()` now supports `| 0 + var` for row faceting (e.g.,
-  `y ~ x | 0 + var`), in addition to the existing `| var` for column faceting.
-
 A lightweight R interface to the
 [AntV G2](https://g2.antv.antgroup.com/) JavaScript visualization library with
 a ggplot2-style API. Create interactive charts using the Grammar of Graphics
diff --git a/R/utils.R b/R/utils.R
index cedfb89..c43ab0a 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -151,13 +151,10 @@ parse_formula = function(f) {
   # Build facet: | var → column; | 0 + var → row; | var1 + var2 → both
   facet = NULL
   if (length(facet_terms)) {
-    enc = list()
-    row_only = facet_terms[1] == '0'
-    if (row_only) facet_terms = facet_terms[-1]
-    if (length(facet_terms) >= 1) {
-      if (row_only) enc$y = facet_terms[1] else enc$x = facet_terms[1]
-    }
-    if (!row_only && length(facet_terms) >= 2) enc$y = facet_terms[2]
+    # A leading 0 shifts terms to row (y) instead of column (x)
+    nms = if (facet_terms[1] == '0') 'y' else c('x', 'y')
+    facet_terms = facet_terms[facet_terms != '0']
+    enc = setNames(as.list(facet_terms), head(nms, length(facet_terms)))
     if (length(enc)) facet = list(type = 'facetRect', encode = enc)
   }
 
diff --git a/vignettes/gglite.Rmd b/vignettes/gglite.Rmd
index 8011024..5e3802f 100644
--- a/vignettes/gglite.Rmd
+++ b/vignettes/gglite.Rmd
@@ -336,6 +336,24 @@ g2(iris, x = 'Sepal.Width', y = 'Sepal.Length', color = 'Species') |>
   facet_rect(x = 'Species')
 ```
 
+The formula interface supports faceting with `|`. Use `| var` for column facets,
+`| 0 + var` for row facets, and `| var1 + var2` for both:
+
+```{r}
+# Column facet: panels arranged in columns by species
+g2(penguins, bill_len ~ bill_dep | species)
+```
+
+```{r}
+# Row facet: panels arranged in rows by island
+g2(penguins, bill_len ~ bill_dep | 0 + island)
+```
+
+```{r}
+# Both: columns by species, rows by island
+g2(penguins, bill_len ~ bill_dep | species + island)
+```
+
 ## Themes
 
 Themes change the overall look. Built-in themes include `theme_classic()`