diff --git a/t/lib-log-graph.sh b/t/lib-log-graph.sh
new file mode 100755
index 0000000000..97cde44dc7
--- /dev/null
+++ b/t/lib-log-graph.sh
@@ -0,0 +1,22 @@
+# Helps shared by the test scripts for comparing log graphs.
+
+sanitize_log_output () {
+	sed -e 's/ *$//' \
+	    -e 's/commit [0-9a-f]*$/commit COMMIT_OBJECT_NAME/' \
+	    -e 's/Merge: [ 0-9a-f]*$/Merge: MERGE_PARENTS/' \
+	    -e 's/Merge tag.*/Merge HEADS DESCRIPTION/' \
+	    -e 's/Merge commit.*/Merge HEADS DESCRIPTION/' \
+	    -e 's/index [0-9a-f]*\.\.[0-9a-f]*/index BEFORE..AFTER/'
+}
+
+lib_test_cmp_graph () {
+	git log --graph "$@" >output &&
+	sed 's/ *$//' >output.sanitized < output &&
+	test_i18ncmp expect output.sanitized
+}
+
+lib_test_cmp_short_graph () {
+	git log --graph --pretty=short "$@" >output &&
+	sanitize_log_output >output.sanitized < output &&
+	test_i18ncmp expect output.sanitized
+}
diff --git a/t/t3430-rebase-merges.sh b/t/t3430-rebase-merges.sh
index e72ca348ea..a1bc3e2001 100755
--- a/t/t3430-rebase-merges.sh
+++ b/t/t3430-rebase-merges.sh
@@ -20,12 +20,11 @@ Initial setup:
 '
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/lib-rebase.sh
+. "$TEST_DIRECTORY"/lib-log-graph.sh
 
 test_cmp_graph () {
 	cat >expect &&
-	git log --graph --boundary --format=%s "$@" >output &&
-	sed "s/ *$//" <output >output.trimmed &&
-	test_cmp expect output.trimmed
+	lib_test_cmp_graph --boundary --format=%s "$@"
 }
 
 test_expect_success 'setup' '
diff --git a/t/t4202-log.sh b/t/t4202-log.sh
index 192347a3e1..e025a9cfc2 100755
--- a/t/t4202-log.sh
+++ b/t/t4202-log.sh
@@ -5,6 +5,11 @@ test_description='git log'
 . ./test-lib.sh
 . "$TEST_DIRECTORY/lib-gpg.sh"
 . "$TEST_DIRECTORY/lib-terminal.sh"
+. "$TEST_DIRECTORY/lib-log-graph.sh"
+
+test_cmp_graph () {
+	lib_test_cmp_graph --format=%s "$@"
+}
 
 test_expect_success setup '
 
@@ -452,8 +457,7 @@ cat > expect <<EOF
 EOF
 
 test_expect_success 'simple log --graph' '
-	git log --graph --pretty=tformat:%s >actual &&
-	test_cmp expect actual
+	test_cmp_graph
 '
 
 cat > expect <<EOF
@@ -467,8 +471,7 @@ cat > expect <<EOF
 EOF
 
 test_expect_success 'simple log --graph --line-prefix="123 "' '
-	git log --graph --line-prefix="123 " --pretty=tformat:%s >actual &&
-	test_cmp expect actual
+	test_cmp_graph --line-prefix="123 "
 '
 
 test_expect_success 'set up merge history' '
@@ -495,9 +498,7 @@ cat > expect <<\EOF
 EOF
 
 test_expect_success 'log --graph with merge' '
-	git log --graph --date-order --pretty=tformat:%s |
-		sed "s/ *\$//" >actual &&
-	test_cmp expect actual
+	test_cmp_graph --date-order
 '
 
 cat > expect <<\EOF
@@ -516,9 +517,7 @@ cat > expect <<\EOF
 EOF
 
 test_expect_success 'log --graph --line-prefix="| | | " with merge' '
-	git log --line-prefix="| | | " --graph --date-order --pretty=tformat:%s |
-		sed "s/ *\$//" >actual &&
-	test_cmp expect actual
+	test_cmp_graph --line-prefix="| | | " --date-order
 '
 
 cat > expect.colors <<\EOF
@@ -676,9 +675,7 @@ cat > expect <<\EOF
 EOF
 
 test_expect_success 'log --graph with merge' '
-	git log --graph --date-order --pretty=tformat:%s |
-		sed "s/ *\$//" >actual &&
-	test_cmp expect actual
+	test_cmp_graph --date-order
 '
 
 test_expect_success 'log.decorate configuration' '
@@ -1213,24 +1210,8 @@ cat >expect <<\EOF
   +one
 EOF
 
-sanitize_output () {
-	sed -e 's/ *$//' \
-	    -e 's/commit [0-9a-f]*$/commit COMMIT_OBJECT_NAME/' \
-	    -e 's/Merge: [ 0-9a-f]*$/Merge: MERGE_PARENTS/' \
-	    -e 's/Merge tag.*/Merge HEADS DESCRIPTION/' \
-	    -e 's/Merge commit.*/Merge HEADS DESCRIPTION/' \
-	    -e 's/, 0 deletions(-)//' \
-	    -e 's/, 0 insertions(+)//' \
-	    -e 's/ 1 files changed, / 1 file changed, /' \
-	    -e 's/, 1 deletions(-)/, 1 deletion(-)/' \
-	    -e 's/, 1 insertions(+)/, 1 insertion(+)/' \
-	    -e 's/index [0-9a-f]*\.\.[0-9a-f]*/index BEFORE..AFTER/'
-}
-
 test_expect_success 'log --graph with diff and stats' '
-	git log --no-renames --graph --pretty=short --stat -p >actual &&
-	sanitize_output >actual.sanitized <actual &&
-	test_i18ncmp expect actual.sanitized
+	lib_test_cmp_short_graph --no-renames --stat -p
 '
 
 cat >expect <<\EOF
@@ -1505,9 +1486,7 @@ cat >expect <<\EOF
 EOF
 
 test_expect_success 'log --line-prefix="*** " --graph with diff and stats' '
-	git log --line-prefix="*** " --no-renames --graph --pretty=short --stat -p >actual &&
-	sanitize_output >actual.sanitized <actual &&
-	test_i18ncmp expect actual.sanitized
+	lib_test_cmp_short_graph --line-prefix="*** " --no-renames --stat -p
 '
 
 cat >expect <<-\EOF
@@ -1529,9 +1508,7 @@ cat >expect <<-\EOF
 EOF
 
 test_expect_success 'log --graph with --name-status' '
-	git log --graph --format=%s --name-status tangle..reach >actual &&
-	sanitize_output <actual >actual.sanitized &&
-	test_cmp expect actual.sanitized
+	test_cmp_graph --name-status tangle..reach
 '
 
 cat >expect <<-\EOF
@@ -1553,9 +1530,7 @@ cat >expect <<-\EOF
 EOF
 
 test_expect_success 'log --graph with --name-only' '
-	git log --graph --format=%s --name-only tangle..reach >actual &&
-	sanitize_output <actual >actual.sanitized &&
-	test_cmp expect actual.sanitized
+	test_cmp_graph --name-only tangle..reach
 '
 
 test_expect_success 'dotdot is a parent directory' '
diff --git a/t/t4214-log-graph-octopus.sh b/t/t4214-log-graph-octopus.sh
index 40d27db674..dedb72ace6 100755
--- a/t/t4214-log-graph-octopus.sh
+++ b/t/t4214-log-graph-octopus.sh
@@ -3,6 +3,12 @@
 test_description='git log --graph of skewed left octopus merge.'
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-log-graph.sh
+
+test_cmp_graph () {
+	cat >expect &&
+	lib_test_cmp_graph --color=never --date-order --format=%s "$@"
+}
 
 test_expect_success 'set up merge history' '
 	test_commit initial &&
@@ -24,7 +30,7 @@ test_expect_success 'set up merge history' '
 '
 
 test_expect_success 'log --graph with tricky octopus merge, no color' '
-	cat >expect.uncolored <<-\EOF &&
+	test_cmp_graph left octopus-merge <<-\EOF
 	* left
 	| *-.   octopus-merge
 	|/|\ \
@@ -37,9 +43,6 @@ test_expect_success 'log --graph with tricky octopus merge, no color' '
 	|/
 	* initial
 	EOF
-	git log --color=never --graph --date-order --pretty=tformat:%s left octopus-merge >actual.raw &&
-	sed "s/ *\$//" actual.raw >actual &&
-	test_cmp expect.uncolored actual
 '
 
 test_expect_success 'log --graph with tricky octopus merge with colors' '
@@ -66,7 +69,7 @@ test_expect_success 'log --graph with tricky octopus merge with colors' '
 # without the first parent skewing to the "left" branch column).
 
 test_expect_success 'log --graph with normal octopus merge, no color' '
-	cat >expect.uncolored <<-\EOF &&
+	test_cmp_graph octopus-merge <<-\EOF
 	*---.   octopus-merge
 	|\ \ \
 	| | | * 4
@@ -78,9 +81,6 @@ test_expect_success 'log --graph with normal octopus merge, no color' '
 	|/
 	* initial
 	EOF
-	git log --color=never --graph --date-order --pretty=tformat:%s octopus-merge >actual.raw &&
-	sed "s/ *\$//" actual.raw >actual &&
-	test_cmp expect.uncolored actual
 '
 
 test_expect_success 'log --graph with normal octopus merge with colors' '
@@ -103,7 +103,7 @@ test_expect_success 'log --graph with normal octopus merge with colors' '
 '
 
 test_expect_success 'log --graph with normal octopus merge and child, no color' '
-	cat >expect.uncolored <<-\EOF &&
+	test_cmp_graph after-merge <<-\EOF
 	* after-merge
 	*---.   octopus-merge
 	|\ \ \
@@ -116,9 +116,6 @@ test_expect_success 'log --graph with normal octopus merge and child, no color'
 	|/
 	* initial
 	EOF
-	git log --color=never --graph --date-order --pretty=tformat:%s after-merge >actual.raw &&
-	sed "s/ *\$//" actual.raw >actual &&
-	test_cmp expect.uncolored actual
 '
 
 test_expect_success 'log --graph with normal octopus and child merge with colors' '
@@ -142,7 +139,7 @@ test_expect_success 'log --graph with normal octopus and child merge with colors
 '
 
 test_expect_success 'log --graph with tricky octopus merge and its child, no color' '
-	cat >expect.uncolored <<-\EOF &&
+	test_cmp_graph left after-merge <<-\EOF
 	* left
 	| * after-merge
 	| *-.   octopus-merge
@@ -156,9 +153,6 @@ test_expect_success 'log --graph with tricky octopus merge and its child, no col
 	|/
 	* initial
 	EOF
-	git log --color=never --graph --date-order --pretty=tformat:%s left after-merge >actual.raw &&
-	sed "s/ *\$//" actual.raw >actual &&
-	test_cmp expect.uncolored actual
 '
 
 test_expect_success 'log --graph with tricky octopus merge and its child with colors' '
@@ -183,7 +177,7 @@ test_expect_success 'log --graph with tricky octopus merge and its child with co
 '
 
 test_expect_success 'log --graph with crossover in octopus merge, no color' '
-	cat >expect.uncolored <<-\EOF &&
+	test_cmp_graph after-4 octopus-merge <<-\EOF
 	* after-4
 	| *---.   octopus-merge
 	| |\ \ \
@@ -200,9 +194,6 @@ test_expect_success 'log --graph with crossover in octopus merge, no color' '
 	|/
 	* initial
 	EOF
-	git log --color=never --graph --date-order --pretty=tformat:%s after-4 octopus-merge >actual.raw &&
-	sed "s/ *\$//" actual.raw >actual &&
-	test_cmp expect.uncolored actual
 '
 
 test_expect_success 'log --graph with crossover in octopus merge with colors' '
@@ -230,7 +221,7 @@ test_expect_success 'log --graph with crossover in octopus merge with colors' '
 '
 
 test_expect_success 'log --graph with crossover in octopus merge and its child, no color' '
-	cat >expect.uncolored <<-\EOF &&
+	test_cmp_graph after-4 after-merge <<-\EOF
 	* after-4
 	| * after-merge
 	| *---.   octopus-merge
@@ -248,9 +239,6 @@ test_expect_success 'log --graph with crossover in octopus merge and its child,
 	|/
 	* initial
 	EOF
-	git log --color=never --graph --date-order --pretty=tformat:%s after-4 after-merge >actual.raw &&
-	sed "s/ *\$//" actual.raw >actual &&
-	test_cmp expect.uncolored actual
 '
 
 test_expect_success 'log --graph with crossover in octopus merge and its child with colors' '
@@ -279,7 +267,7 @@ test_expect_success 'log --graph with crossover in octopus merge and its child w
 '
 
 test_expect_success 'log --graph with unrelated commit and octopus tip, no color' '
-	cat >expect.uncolored <<-\EOF &&
+	test_cmp_graph after-initial octopus-merge <<-\EOF
 	* after-initial
 	| *---.   octopus-merge
 	| |\ \ \
@@ -296,9 +284,6 @@ test_expect_success 'log --graph with unrelated commit and octopus tip, no color
 	|/
 	* initial
 	EOF
-	git log --color=never --graph --date-order --pretty=tformat:%s after-initial octopus-merge >actual.raw &&
-	sed "s/ *\$//" actual.raw >actual &&
-	test_cmp expect.uncolored actual
 '
 
 test_expect_success 'log --graph with unrelated commit and octopus tip with colors' '
@@ -326,7 +311,7 @@ test_expect_success 'log --graph with unrelated commit and octopus tip with colo
 '
 
 test_expect_success 'log --graph with unrelated commit and octopus child, no color' '
-	cat >expect.uncolored <<-\EOF &&
+	test_cmp_graph after-initial after-merge <<-\EOF
 	* after-initial
 	| * after-merge
 	| *---.   octopus-merge
@@ -344,9 +329,6 @@ test_expect_success 'log --graph with unrelated commit and octopus child, no col
 	|/
 	* initial
 	EOF
-	git log --color=never --graph --date-order --pretty=tformat:%s after-initial after-merge >actual.raw &&
-	sed "s/ *\$//" actual.raw >actual &&
-	test_cmp expect.uncolored actual
 '
 
 test_expect_success 'log --graph with unrelated commit and octopus child with colors' '
diff --git a/t/t4215-log-skewed-merges.sh b/t/t4215-log-skewed-merges.sh
index 1d0d3240ff..e1e94176da 100755
--- a/t/t4215-log-skewed-merges.sh
+++ b/t/t4215-log-skewed-merges.sh
@@ -3,12 +3,11 @@
 test_description='git log --graph of skewed merges'
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-log-graph.sh
 
 check_graph () {
 	cat >expect &&
-	git log --graph --pretty=tformat:%s "$@" >actual.raw &&
-	sed "s/ *$//" actual.raw >actual &&
-	test_cmp expect actual
+	lib_test_cmp_graph --format=%s "$@"
 }
 
 test_expect_success 'log --graph with merge fusing with its left and right neighbors' '
