fix(tooltip): Fix for dynamically loaded data adding column

Correct getting nullable index value

Fix #660
Close #661

spec/internals/tooltip-spec.js

@@ -675,4 +675,37 @@ describe("TOOLTIP", function() {
 			expect(value).to.be.equal(200);
 		});
 	});
+
+	describe("tooltip for dynamic loaded data", () => {
+		before(() => {
+			args = {
+				data: {
+					columns: [
+						["data1", 30, 200, 100],
+						["data2", 130, 100, 140]
+					]
+				}
+			};
+		});
+
+		it("load data to be adding more columns", done => {
+			setTimeout(() => {
+				chart.load({
+					columns: [
+						["data2", 44, 134, 98, 170]
+					],
+					done: () => {
+						try {
+							chart.tooltip.show({index: 3});
+						} catch(e) {
+							expect(false).to.be.true;
+						}
+
+						expect(true).to.be.true;
+						done();
+					}
+				});
+			}, 500);
+		});
+	});
 });

src/internals/tooltip.js

@@ -233,7 +233,7 @@ extend(ChartInternal.prototype, {
 		let height = (datum && datum.height) || 0;
 
 		if (!datum || datum.current !== dataStr) {
-			const index = selectedData[0].index;
+			const index = selectedData.concat().sort()[0].index;
 			const html = config.tooltip_contents.call(
 				$$,
 				selectedData,