Allow to set start date

Before, heatmap always ends today and starts a year ago. Now a
startDate can be set by the user via .startData(Date)

README.md

@@ -19,6 +19,7 @@ A [d3.js](https://d3js.org/) heatmap representing time series data. Inspired by
 | data | Chart data | none | yes |
 | selector | DOM selector to attach the chart to | body | no |
 | max | Maximum count | max found in data | no |
+| startDate | Date to start heatmap at | 1 year ago | no |
 | colorRange | Minimum and maximum chart gradient colors | ['#D8E6E7', '#218380'] | no |
 | tooltipEnabled | Option to render a tooltip | true | no |
 | tooltipUnit | Unit to render on the tooltip | 'contributions' | no |

src/calendar-heatmap.js

@@ -12,6 +12,7 @@ function calendarHeatmap() {
   var MONTH_LABEL_PADDING = 6;
   var now = moment().endOf('day').toDate();
   var yearAgo = moment().startOf('day').subtract(1, 'year').toDate();
+  var startDate = null;
   var data = [];
   var max = null;
   var colorRange = ['#D8E6E7', '#218380'];
@@ -40,6 +41,13 @@ function calendarHeatmap() {
     return chart;
   };
 
+  chart.startDate = function (value) {
+    if (!arguments.length) { return startDate; }
+    yearAgo = value;
+    now = moment(value).endOf('day').add(1, 'year').toDate();
+    return chart;
+  };
+
   chart.colorRange = function (value) {
     if (!arguments.length) { return colorRange; }
     colorRange = value;