Added example of setting a watermark in Excel.

test/functional/src/test_header_image20.c

@@ -0,0 +1,22 @@
+/*****************************************************************************
+ * Test cases for libxlsxwriter.
+ *
+ * Test to compare output against Excel files.
+ *
+ * Copyright 2014-2022, John McNamara, jmcnamara@cpan.org
+ *
+ */
+
+#include "xlsxwriter.h"
+
+int main() {
+
+    lxw_workbook  *workbook  = workbook_new("test_header_image20.xlsx");
+    lxw_worksheet *worksheet = workbook_add_worksheet(workbook, NULL);
+
+    lxw_header_footer_options header_options = {.image_center = "images/watermark.png"};
+
+    worksheet_set_header_opt(worksheet, "&C&G", &header_options);
+
+    return workbook_close(workbook);
+}

test/functional/test_header_image.py

@@ -100,6 +100,10 @@ class TestCompareXLSXFiles(base_test_class.XLSXBaseTest):
         self.ignore_elements = {'xl/worksheets/sheet1.xml': ['<pageMargins', '<pageSetup']}
         self.run_exe_test('test_header_image19')
 
+    def test_header_image20(self):
+        self.ignore_elements = {'xl/worksheets/sheet1.xml': ['<pageMargins', '<pageSetup']}
+        self.run_exe_test('test_header_image20')
+                          
     # Test format strings with &[Picture] instead of &G.
     def test_header_image51(self):
         self.ignore_elements = {'xl/worksheets/sheet1.xml': ['<pageMargins', '<pageSetup']}