Add axis interval support.

include/xlsxwriter/chart.h

@@ -700,6 +700,9 @@ typedef struct lxw_chart_axis {
     uint8_t has_minor_unit;
     double minor_unit;
 
+    uint16_t interval_unit;
+    uint16_t interval_tick;
+
     uint16_t log_base;
 
 } lxw_chart_axis;
@@ -1529,6 +1532,54 @@ void chart_axis_set_max(lxw_chart_axis *axis, double max);
  */
 void chart_axis_set_log_base(lxw_chart_axis *axis, uint16_t log_base);
 
+/**
+ * @brief Set the interval between category values.
+ *
+ * @param axis A pointer to a chart #lxw_chart_axis object.
+ * @param unit The interval between the categories.
+ *
+ * Set the interval between the category values. The default interval is 1
+ * which gives the intervals shown in the charts above:
+ *
+ *     1, 2, 3, 4, 5, etc.
+ *
+ * Setting it to 2 gives:
+ *
+ *     1, 3, 5, 7, etc.
+ *
+ * For example:
+ *
+ * @code
+ *     chart_axis_set_interval_unit(chart->x_axis, 2);
+ * @endcode
+ *
+ * @image html chart_set_interval1.png
+ *
+ * **Axis types**: This function is applicable to category and date axes only.
+ *                 See @ref ww_charts_axes.
+ */
+void chart_axis_set_interval_unit(lxw_chart_axis *axis, uint16_t unit);
+
+/**
+ * @brief Set the interval between category tick marks.
+ *
+ * @param axis A pointer to a chart #lxw_chart_axis object.
+ * @param unit The interval between the category ticks.
+ *
+ * Set the interval between the category tick marks. The default interval is 1
+ * between each category but it can be set to other integer values:
+ *
+ * @code
+ *     chart_axis_set_interval_tick(chart->x_axis, 2);
+ * @endcode
+ *
+ * @image html chart_set_interval2.png
+ *
+ * **Axis types**: This function is applicable to category and date axes only.
+ *                 See @ref ww_charts_axes.
+ */
+void chart_axis_set_interval_tick(lxw_chart_axis *axis, uint16_t unit);
+
 /**
  * @brief Set the increment of the major units in the axis.
  *

src/chart.c

@@ -2333,6 +2333,46 @@ _chart_write_label_align(lxw_chart *self)
     LXW_FREE_ATTRIBUTES();
 }
 
+/*
+ * Write the <c:tickLblSkip> element.
+ */
+STATIC void
+_chart_write_tick_label_skip(lxw_chart *self, lxw_chart_axis *axis)
+{
+    struct xml_attribute_list attributes;
+    struct xml_attribute *attribute;
+
+    if (!axis->interval_unit)
+        return;
+
+    LXW_INIT_ATTRIBUTES();
+    LXW_PUSH_ATTRIBUTES_INT("val", axis->interval_unit);
+
+    lxw_xml_empty_tag(self->file, "c:tickLblSkip", &attributes);
+
+    LXW_FREE_ATTRIBUTES();
+}
+
+/*
+ * Write the <c:tickMarkSkip> element.
+ */
+STATIC void
+_chart_write_tick_mark_skip(lxw_chart *self, lxw_chart_axis *axis)
+{
+    struct xml_attribute_list attributes;
+    struct xml_attribute *attribute;
+
+    if (!axis->interval_tick)
+        return;
+
+    LXW_INIT_ATTRIBUTES();
+    LXW_PUSH_ATTRIBUTES_INT("val", axis->interval_tick);
+
+    lxw_xml_empty_tag(self->file, "c:tickMarkSkip", &attributes);
+
+    LXW_FREE_ATTRIBUTES();
+}
+
 /*
  * Write the <c:majorUnit> element.
  */
@@ -2796,6 +2836,12 @@ _chart_write_cat_axis(lxw_chart *self)
     /* Write the c:lblOffset element. */
     _chart_write_label_offset(self);
 
+    /* Write the c:tickLblSkip element. */
+    _chart_write_tick_label_skip(self, self->x_axis);
+
+    /* Write the c:tickMarkSkip element. */
+    _chart_write_tick_mark_skip(self, self->x_axis);
+
     lxw_xml_end_tag(self->file, "c:catAx");
 }
 
@@ -4048,6 +4094,28 @@ chart_axis_set_log_base(lxw_chart_axis *axis, uint16_t log_base)
         axis->log_base = log_base;
 }
 
+/*
+ * Set interval unit for a category axis.
+ */
+void
+chart_axis_set_interval_unit(lxw_chart_axis *axis, uint16_t unit)
+{
+    LXW_WARN_CAT_AND_DATE_AXIS_ONLY("chart_axis_set_major_unit");
+
+    axis->interval_unit = unit;
+}
+
+/*
+ * Set tick interval for a category axis.
+ */
+void
+chart_axis_set_interval_tick(lxw_chart_axis *axis, uint16_t unit)
+{
+    LXW_WARN_CAT_AND_DATE_AXIS_ONLY("chart_axis_set_major_tick");
+
+    axis->interval_tick = unit;
+}
+
 /*
  * Set major unit for a value axis.
  */

test/functional/src/test_chart_axis34.c

@@ -0,0 +1,44 @@
+/*****************************************************************************
+ * Test cases for libxlsxwriter.
+ *
+ * Test to compare output against Excel files.
+ *
+ * Copyright 2014-2017, John McNamara, jmcnamara@cpan.org
+ *
+ */
+
+#include "xlsxwriter.h"
+
+int main() {
+
+    lxw_workbook  *workbook  = new_workbook("test_chart_axis34.xlsx");
+    lxw_worksheet *worksheet = workbook_add_worksheet(workbook, NULL);
+    lxw_chart     *chart     = workbook_add_chart(workbook, LXW_CHART_LINE);
+
+    /* For testing, copy the randomly generated axis ids in the target file. */
+    chart->axis_id_1 = 54712192;
+    chart->axis_id_2 = 54713728;
+
+    uint8_t data[5][3] = {
+        {1, 2,  3},
+        {2, 4,  6},
+        {3, 6,  9},
+        {4, 8,  12},
+        {5, 10, 15}
+    };
+
+    int row, col;
+    for (row = 0; row < 5; row++)
+        for (col = 0; col < 3; col++)
+            worksheet_write_number(worksheet, row, col, data[row][col], NULL);
+
+    chart_add_series(chart, NULL, "=Sheet1!$A$1:$A$5");
+    chart_add_series(chart, NULL, "=Sheet1!$B$1:$B$5");
+    chart_add_series(chart, NULL, "=Sheet1!$C$1:$C$5");
+
+    chart_axis_set_interval_unit(chart->x_axis, 2);
+
+    worksheet_insert_chart(worksheet, CELL("E9"), chart);
+
+    return workbook_close(workbook);
+}

test/functional/src/test_chart_axis40.c

@@ -0,0 +1,45 @@
+/*****************************************************************************
+ * Test cases for libxlsxwriter.
+ *
+ * Test to compare output against Excel files.
+ *
+ * Copyright 2014-2017, John McNamara, jmcnamara@cpan.org
+ *
+ */
+
+#include "xlsxwriter.h"
+
+int main() {
+
+    lxw_workbook  *workbook  = new_workbook("test_chart_axis40.xlsx");
+    lxw_worksheet *worksheet = workbook_add_worksheet(workbook, NULL);
+    lxw_chart     *chart     = workbook_add_chart(workbook, LXW_CHART_COLUMN);
+
+    /* For testing, copy the randomly generated axis ids in the target file. */
+    chart->axis_id_1 = 108329216;
+    chart->axis_id_2 = 108635264;
+
+    uint8_t data[5][3] = {
+        {1, 2,  3},
+        {2, 4,  6},
+        {3, 6,  9},
+        {4, 8,  12},
+        {5, 10, 15}
+    };
+
+    int row, col;
+    for (row = 0; row < 5; row++)
+        for (col = 0; col < 3; col++)
+            worksheet_write_number(worksheet, row, col, data[row][col], NULL);
+
+    chart_add_series(chart, NULL, "=Sheet1!$A$1:$A$5");
+    chart_add_series(chart, NULL, "=Sheet1!$B$1:$B$5");
+    chart_add_series(chart, NULL, "=Sheet1!$C$1:$C$5");
+
+    chart_axis_set_interval_unit(chart->x_axis, 3);
+    chart_axis_set_interval_tick(chart->x_axis, 2);
+
+    worksheet_insert_chart(worksheet, CELL("E9"), chart);
+
+    return workbook_close(workbook);
+}

test/functional/test_chart_axis.py

@@ -94,6 +94,9 @@ class TestCompareXLSXFiles(base_test_class.XLSXBaseTest):
     def test_chart_axis33(self):
         self.run_exe_test('test_chart_axis33')
 
+    def test_chart_axis34(self):
+        self.run_exe_test('test_chart_axis34')
+
     def test_chart_axis35(self):
         self.run_exe_test('test_chart_axis35')
 
@@ -108,3 +111,6 @@ class TestCompareXLSXFiles(base_test_class.XLSXBaseTest):
 
     def test_chart_axis39(self):
         self.run_exe_test('test_chart_axis39')
+
+    def test_chart_axis40(self):
+        self.run_exe_test('test_chart_axis40')