gh-1595 Adds config option in area hash to draw the area fill above the line instead of below

src/config.js

@@ -168,6 +168,7 @@ c3_chart_internal_fn.getDefaultConfig = function () {
         bar_zerobased: true,
         // area
         area_zerobased: true,
+        area_above: false,
         // pie
         pie_label_show: true,
         pie_label_format: undefined,

src/shape.line.js

@@ -250,7 +250,7 @@ c3_chart_internal_fn.generateDrawArea = function (areaIndices, isSub) {
             return config.data_groups.length > 0 ? getPoints(d, i)[1][1] : yScaleGetter.call($$, d.id)(d.value);
         };
 
-    area = config.axis_rotated ? area.x0(value0).x1(value1).y(xValue) : area.x(xValue).y0(value0).y1(value1);
+    area = config.axis_rotated ? area.x0(value0).x1(value1).y(xValue) : area.x(xValue).y0(config.area_above ? 0 : value0).y1(value1);
     if (!config.line_connectNull) {
         area = area.defined(function (d) { return d.value !== null; });
     }