Add axis.xFormat axis.yFormat API

c3.js

@@ -7930,6 +7930,31 @@
             };
         }
     };
+
+    c3_chart_fn.axis.xFormat = function(value){
+        var $$ = this.internal, config = $$.config;
+
+        if(value === undefined){
+            return config.axis_x_tick_format;
+        }
+
+        config.axis_x_tick_format = value;
+
+        $$.redraw();
+    };
+
+    c3_chart_fn.axis.yFormat = function(value){
+        var $$ = this.internal, config = $$.config;
+
+        if(value === undefined){
+            return config.axis_y_tick_format;
+        }
+
+        config.axis_y_tick_format = value;
+
+        $$.redraw();
+    };
+
     c3_chart_fn.axisX = function (value){
         var $$ = this.internal;
         
@@ -8325,7 +8350,7 @@
                 text.style('display', function(d, i){
                     return shouldDrawTickText(i - indexOfZero) ? 'block' : 'none';
                 });
-                
+
                 tspan = text.selectAll('tspan')
                     .data(function (d, i) {
                         var splitted = params.tickMultiline ? splitTickText(d, params.tickWidth) : [].concat(textFormatted(d));

spec/axis-spec.js

@@ -736,4 +736,54 @@ describe('c3 chart axis', function () {
 
     });
 
+    describe('chart.axis.xFormat', function(){
+        it('should update args', function(){
+            args = {
+                data: {
+                    columns: [
+                        ['data1', 0, 1, 2, 3]
+                    ]
+                },
+                axis: {
+                    x: {
+                        show: true
+                    }, 
+                    y: {
+                        show: true
+                    }
+                }
+            };
+
+            expect(true).toBeTruthy();
+        });
+
+        it('should update x format function', function(){
+            function format(x){
+                return x+10;
+            }
+
+            chart.axis.xFormat(format);
+            var tickSpans = chart.internal.main.selectAll('.c3-axis-x g.tick text tspan')[0];
+
+            for(var i = 0; i < tickSpans.length; i++){
+                expect(tickSpans[i].textContent).toBe('' + (i+10)); 
+            }
+        });
+
+        it('should update y format function', function(){
+            function format(x){
+                return x-10;
+            }
+
+            chart.axis.yFormat(format);
+
+            var tickSpans = chart.internal.main.selectAll('.c3-axis-y g.tick text tspan')[0];
+            var expected = ['-10', '-9.5', '-9', '-8.5', '-8', '-7.5', '-7'];
+
+            for(var i = 0; i < tickSpans.length; i++){
+                expect(tickSpans[i].textContent).toBe(expected[i]); 
+            }
+        });
+    });
+
 });

src/api.axis.js

@@ -58,6 +58,31 @@ c3_chart_fn.axis.range = function (range) {
         };
     }
 };
+
+c3_chart_fn.axis.xFormat = function(value){
+    var $$ = this.internal, config = $$.config;
+
+    if(value === undefined){
+        return config.axis_x_tick_format;
+    }
+
+    config.axis_x_tick_format = value;
+
+    $$.redraw();
+};
+
+c3_chart_fn.axis.yFormat = function(value){
+    var $$ = this.internal, config = $$.config;
+
+    if(value === undefined){
+        return config.axis_y_tick_format;
+    }
+
+    config.axis_y_tick_format = value;
+
+    $$.redraw();
+};
+
 c3_chart_fn.axisX = function (value){
     var $$ = this.internal;
     

src/c3.axis.js

@@ -220,7 +220,7 @@ function c3_axis(d3, params, $$) {
             text.style('display', function(d, i){
                 return shouldDrawTickText(i - indexOfZero) ? 'block' : 'none';
             });
-            
+
             tspan = text.selectAll('tspan')
                 .data(function (d, i) {
                     var splitted = params.tickMultiline ? splitTickText(d, params.tickWidth) : [].concat(textFormatted(d));