Fix data label position for cero value when all the data set is negative

When all the data set is negative the label position of cero value will
be above the the y axis line then you will not see the label.

c3.css

@@ -70,11 +70,11 @@
 /*-- Region --*/
 .c3-region {
   fill: steelblue;
-  fill-opacity: 0.1; }
+  fill-opacity: .1; }
 
 /*-- Brush --*/
 .c3-brush .extent {
-  fill-opacity: 0.1; }
+  fill-opacity: .1; }
 
 /*-- Select - Drag --*/
 /*-- Legend --*/

c3.js

@@ -2038,6 +2038,8 @@
             xs = $$.d3.keys(data[0]).filter($$.isX, $$),
             targets;
 
+        $$.allDataIsNegative = true;
+
         // save x for update data by load when custom x and c3.x API
         ids.forEach(function (id) {
             var xKey = $$.getXKey(id);
@@ -2080,7 +2082,9 @@
                 id: convertedId,
                 id_org: id,
                 values: data.map(function (d, i) {
-                    var xKey = $$.getXKey(id), rawX = d[xKey], x = $$.generateTargetX(rawX, id, i);
+                    var xKey = $$.getXKey(id), rawX = d[xKey],
+                        x = $$.generateTargetX(rawX, id, i),
+                        value = d[id] !== null && !isNaN(d[id]) ? +d[id] : null;
                     // use x as categories if custom x and categorized
                     if ($$.isCustomX() && $$.isCategorized() && index === 0 && rawX) {
                         if (i === 0) { config.axis_x_categories = []; }
@@ -2090,7 +2094,12 @@
                     if (isUndefined(d[id]) || $$.data.xs[id].length <= i) {
                         x = undefined;
                     }
-                    return {x: x, value: d[id] !== null && !isNaN(d[id]) ? +d[id] : null, id: convertedId};
+
+                    if(value > 0) {
+                        $$.allDataIsNegative = false;
+                    }
+
+                    return {x: x, value: value, id: convertedId};
                 }).filter(function (v) { return isDefined(v.x); })
             };
         });
@@ -3352,7 +3361,7 @@
             yPos = (points[0][0] + points[2][0] + box.height * 0.6) / 2;
         } else {
             yPos = points[2][1];
-            if (d.value < 0) {
+            if (d.value < 0  || (d.value === 0 && $$.allDataIsNegative)) {
                 yPos += box.height;
                 if ($$.isBarType(d) && $$.isSafari()) {
                     yPos -= 3;

src/data.convert.js

@@ -98,6 +98,8 @@ c3_chart_internal_fn.convertDataToTargets = function (data, appendXs) {
         xs = $$.d3.keys(data[0]).filter($$.isX, $$),
         targets;
 
+    $$.allDataIsNegative = true;
+
     // save x for update data by load when custom x and c3.x API
     ids.forEach(function (id) {
         var xKey = $$.getXKey(id);
@@ -140,7 +142,9 @@ c3_chart_internal_fn.convertDataToTargets = function (data, appendXs) {
             id: convertedId,
             id_org: id,
             values: data.map(function (d, i) {
-                var xKey = $$.getXKey(id), rawX = d[xKey], x = $$.generateTargetX(rawX, id, i);
+                var xKey = $$.getXKey(id), rawX = d[xKey],
+                    x = $$.generateTargetX(rawX, id, i),
+                    value = d[id] !== null && !isNaN(d[id]) ? +d[id] : null;
                 // use x as categories if custom x and categorized
                 if ($$.isCustomX() && $$.isCategorized() && index === 0 && rawX) {
                     if (i === 0) { config.axis_x_categories = []; }
@@ -150,7 +154,12 @@ c3_chart_internal_fn.convertDataToTargets = function (data, appendXs) {
                 if (isUndefined(d[id]) || $$.data.xs[id].length <= i) {
                     x = undefined;
                 }
-                return {x: x, value: d[id] !== null && !isNaN(d[id]) ? +d[id] : null, id: convertedId};
+
+                if(value > 0) {
+                    $$.allDataIsNegative = false;
+                }
+
+                return {x: x, value: value, id: convertedId};
             }).filter(function (v) { return isDefined(v.x); })
         };
     });

src/text.js

@@ -98,7 +98,7 @@ c3_chart_internal_fn.getYForText = function (points, d, textElement) {
         yPos = (points[0][0] + points[2][0] + box.height * 0.6) / 2;
     } else {
         yPos = points[2][1];
-        if (d.value < 0) {
+        if (d.value < 0  || (d.value === 0 && $$.allDataIsNegative)) {
             yPos += box.height;
             if ($$.isBarType(d) && $$.isSafari()) {
                 yPos -= 3;