Allow to set max value manually

Setting max value manually allows to compare calendar heatmaps
absolutely, rather than be based on the maximum value in the data.

Call function with .max(int)

README.md

@@ -18,6 +18,7 @@ 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 |
 | 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

@@ -13,6 +13,7 @@ function calendarHeatmap() {
   var now = moment().endOf('day').toDate();
   var yearAgo = moment().startOf('day').subtract(1, 'year').toDate();
   var data = [];
+  var max = null;
   var colorRange = ['#D8E6E7', '#218380'];
   var tooltipEnabled = true;
   var tooltipUnit = 'contribution';
@@ -27,6 +28,12 @@ function calendarHeatmap() {
     return chart;
   };
 
+  chart.max = function (value) {
+    if (!arguments.length) { return max; }
+    max = value;
+    return chart;
+  };
+
   chart.selector = function (value) {
     if (!arguments.length) { return selector; }
     selector = value;
@@ -70,7 +77,7 @@ function calendarHeatmap() {
     var dateRange = d3.time.days(yearAgo, now); // generates an array of date objects within the specified range
     var monthRange = d3.time.months(moment(yearAgo).startOf('month').toDate(), now); // it ignores the first month if the 1st date is after the start of the month
     var firstDate = moment(dateRange[0]);
-    var max = d3.max(chart.data(), function (d) { return d.count; }); // max data value
+    if (max === null) { max = d3.max(chart.data(), function (d) { return d.count; }); } // max data value
 
     // color range
     var color = d3.scale.linear()