Adding invert method to ordinal scale

src/utils/scales-utils.js

@@ -150,6 +150,37 @@ export function _getSmallestDistanceIndex(values, scaleObject) {
   return result;
 }
 
+/**
+ * This is a workaround for issue that ordinal scale
+ * does not have invert method implemented in d3-scale.
+ * @param {Object} Ordinal d3-scale object.
+ * @returns {void}
+ * @private
+ */
+
+function addInvertFunctionToOrdinalScaleObject(scale) {
+  if (scale.invert) { 
+    return;
+  }
+
+  scale.invert = function invert(value) {
+    const [lower, upper] = scale.range();
+    const start = Math.min(lower, upper);
+    const stop = Math.max(lower, upper);
+
+    if (value < start + scale.padding() * scale.step()) {
+      return scale.domain()[0];
+    } 
+
+    if (value > stop - scale.padding() * scale.step()) { 
+      return scale.domain()[scale.domain().length - 1];
+    }
+
+    const index = Math.floor((value - start - scale.padding() * scale.step()) / scale.step());
+    return scale.domain()[index];
+  }
+}
+
 /**
  * Crate a scale function from the scale object.
  * @param {Object} scaleObject Scale object.
@@ -179,6 +210,7 @@ export function getScaleFnFromScaleObject(scaleObject) {
     .range(range);
   if (type === ORDINAL_SCALE_TYPE) {
     scale.padding(0.5);
+	  addInvertFunctionToOrdinalScaleObject(scale);
   }
   return scale;
 }

tests/utils/scales-utils-tests.js

@@ -505,6 +505,22 @@ test('scales-utils #getScaleFnFromScaleObject', t => {
     [-1, 1],
     'should build a generic domain that reflects about zero'
   );
+
+  const ordinalScale = getScaleFnFromScaleObject({
+    type: 'ordinal',
+    domain: ["a", "b", "c", "d", "e"],
+    range: [20, 120]
+  });
+
+  t.equal(ordinalScale.invert(-10), "a");
+  t.equal(ordinalScale.invert(25), "a");
+  t.equal(ordinalScale.invert(40), "a");
+  t.equal(ordinalScale.invert(60), "b");
+  t.equal(ordinalScale.invert(80), "c");
+  t.equal(ordinalScale.invert(100), "d");
+  t.equal(ordinalScale.invert(115), "e");
+  t.equal(ordinalScale.invert(130), "e");
+
   t.end();
 });