add half-year format directive

src/lib/dates.js

@@ -370,11 +370,19 @@ exports.cleanDate = function(v, dflt, calendar) {
  */
 
 /*
- * modDateFormat: Support world calendars, and add one item to
+ * modDateFormat: Support world calendars, and add two items to
  * d3's vocabulary:
  * %{n}f where n is the max number of digits of fractional seconds
+ * %h formats: half of the year as a decimal number [1,2]
  */
 var fracMatch = /%\d?f/g;
+var halfYearMatch = /%h/g;
+var quarterToHalfYear = {
+    '1': 1,
+    '2': 1,
+    '3': 2,
+    '4': 2,
+};
 function modDateFormat(fmt, x, formatter, calendar) {
     fmt = fmt.replace(fracMatch, function(match) {
         var digits = Math.min(+(match.charAt(1)) || 6, 6);
@@ -386,6 +394,10 @@ function modDateFormat(fmt, x, formatter, calendar) {
 
     var d = new Date(Math.floor(x + 0.05));
 
+    fmt = fmt.replace(halfYearMatch, function() {
+        return quarterToHalfYear[formatter('%q')(d)];
+    });
+
     if(isWorldCalendar(calendar)) {
         try {
             fmt = Registry.getComponentMethod('calendars', 'worldCalFmt')(fmt, x, calendar);

test/image/mocks/period_positioning9.json

@@ -0,0 +1,33 @@
+{
+    "data": [
+        {
+            "xperiod": "M6",
+            "name": "Half year",
+            "type": "bar",
+            "y": [1, 2, 3, 4, 5, 6],
+            "x": ["2001-01", "2001-07", "2002-01", "2002-07", "2003-01", "2003-07"],
+            "texttemplate": "%{x}",
+            "textangle": 0
+        }
+    ],
+    "layout": {
+        "title": {
+            "text": "half-year periods and tickformat"
+        },
+        "width": 400,
+        "height": 300,
+        "margin": {
+            "t": 50,
+            "b": 50,
+            "l": 25,
+            "r": 25
+        },
+
+        "xaxis": {
+            "dtick": "M6",
+            "tickformat": "%Y H%h",
+            "ticklabelmode": "period",
+            "tickcolor": "black"
+        }
+    }
+}