Added a 4th param to data label format callback

This gives us more control of data labels. Now we have access to: value,
id, i (index of each datum inside a series), j (index of each series).

c3.js

@@ -3127,7 +3127,7 @@
             .style("fill", function (d) { return $$.color(d); })
             .style("fill-opacity", 0);
         $$.mainText
-            .text(function (d, i) { return $$.formatByAxisId($$.getAxisId(d.id))(d.value, d.id, i); });
+            .text(function (d, i, j) { return $$.formatByAxisId($$.getAxisId(d.id))(d.value, d.id, i, j); });
         $$.mainText.exit()
             .transition().duration(durationForExit)
             .style('fill-opacity', 0)

src/text.js

@@ -32,7 +32,7 @@ c3_chart_internal_fn.redrawText = function (durationForExit) {
         .style("fill", function (d) { return $$.color(d); })
         .style("fill-opacity", 0);
     $$.mainText
-        .text(function (d, i) { return $$.formatByAxisId($$.getAxisId(d.id))(d.value, d.id, i); });
+        .text(function (d, i, j) { return $$.formatByAxisId($$.getAxisId(d.id))(d.value, d.id, i, j); });
     $$.mainText.exit()
         .transition().duration(durationForExit)
         .style('fill-opacity', 0)