From 77f57e1f8fbef40120cde8b78359c1e5600ba0cf Mon Sep 17 00:00:00 2001
From: Manuel Rubio <manuel@altenwald.com>
Date: Thu, 25 Jan 2024 19:26:45 +0100
Subject: [PATCH 1/3] fix zero and negative value for grouped barcharts

---
 lib/chart/barchart.ex | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/lib/chart/barchart.ex b/lib/chart/barchart.ex
index a9f2209..6d8ef6b 100644
--- a/lib/chart/barchart.ex
+++ b/lib/chart/barchart.ex
@@ -504,12 +504,18 @@ defmodule Contex.BarChart do
   end
 
   defp prepare_bar_values(series_values, scale, :grouped) do
-    {scale_min, _} = Scale.get_range(scale)
+    scale_zero = Scale.domain_to_range(scale, 0.0)
 
     results =
       Enum.reduce(series_values, [], fn data_val, points ->
         range_val = Scale.domain_to_range(scale, data_val)
-        [{scale_min, range_val} | points]
+        cond do
+          data_val != 0 ->
+            [{scale_zero, range_val} | points]
+
+          :else ->
+            [{scale_zero, scale_zero} | points]
+        end
       end)
 
     Enum.reverse(results)

From 7a35a3de423b23fd8fee57cea2f18c8660f54b90 Mon Sep 17 00:00:00 2001
From: Manuel Rubio <manuel@altenwald.com>
Date: Sun, 4 Feb 2024 22:01:04 +0100
Subject: [PATCH 2/3] remove unclear code and keep it simpler for setting zero
 and negative values for grouped barcharts

---
 lib/chart/barchart.ex | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/lib/chart/barchart.ex b/lib/chart/barchart.ex
index 6d8ef6b..fe64b77 100644
--- a/lib/chart/barchart.ex
+++ b/lib/chart/barchart.ex
@@ -509,13 +509,7 @@ defmodule Contex.BarChart do
     results =
       Enum.reduce(series_values, [], fn data_val, points ->
         range_val = Scale.domain_to_range(scale, data_val)
-        cond do
-          data_val != 0 ->
-            [{scale_zero, range_val} | points]
-
-          :else ->
-            [{scale_zero, scale_zero} | points]
-        end
+        [{scale_zero, range_val} | points]
       end)
 
     Enum.reverse(results)

From 3563ba5224b2c1d1d238468ab0ac017a60b6c6a0 Mon Sep 17 00:00:00 2001
From: Manuel Rubio <manuel@altenwald.com>
Date: Tue, 6 Feb 2024 09:02:31 +0100
Subject: [PATCH 3/3] zero axis (as @mindok suggested)

---
 lib/chart/axis.ex     | 11 +++++++++++
 lib/chart/barchart.ex |  3 +++
 2 files changed, 14 insertions(+)

diff --git a/lib/chart/axis.ex b/lib/chart/axis.ex
index 615fff7..0883e53 100644
--- a/lib/chart/axis.ex
+++ b/lib/chart/axis.ex
@@ -131,6 +131,17 @@ defmodule Contex.Axis do
     ]
   end
 
+  @doc """
+  Draw the zero line for ensuring it's drawn even when zero
+  isn't aligned to the base of the graph.
+  """
+  def get_zero_line_svg(%Axis{scale: scale} = axis) do
+    # Check that scale intersects zero by looking at domain
+    # If so...
+   domain_to_range_fn = Scale.domain_to_range_fn(scale)
+   get_svg_gridline(axis, domain_to_range_fn.(0.0))
+  end
+
   defp get_svg_gridlines(%Axis{scale: scale} = axis) do
     domain_ticks = Scale.ticks_domain(scale)
     domain_to_range_fn = Scale.domain_to_range_fn(scale)
diff --git a/lib/chart/barchart.ex b/lib/chart/barchart.ex
index fe64b77..16668c5 100644
--- a/lib/chart/barchart.ex
+++ b/lib/chart/barchart.ex
@@ -357,9 +357,12 @@ defmodule Contex.BarChart do
 
     val_axis_svg = if plot_options.show_val_axis, do: Axis.to_svg(value_axis), else: ""
 
+    zero_line = Axis.get_zero_line_svg(category_axis)
+
     [
       cat_axis_svg,
       val_axis_svg,
+      zero_line,
       "<g>",
       get_svg_bars(plot),
       "</g>"