refactor(grid): update threshold for grid interval (#2812)

Adjust interval for grid lines display when grid.y.ticks is given

src/ChartInternal/Axis/AxisRenderer.ts

@@ -243,7 +243,7 @@ export default class AxisRenderer {
 		let res = this.generatedTicks;
 
 		if (len > count) {
-			const interval = Math.round((len / count) - 0.1);
+			const interval = Math.round((len / count) + 0.1);
 
 			res = this.generatedTicks
 				.map((v, i) => (i % interval === 0 ? v : null))

test/internals/grid-spec.ts

@@ -177,6 +177,36 @@ describe("GRID", function() {
 			});
 		});
 
+		it("set options", () => {
+				args = {
+					data: {
+						columns: [
+							["data1", 130, 340, 200, 500, 250, 350]
+						],
+						type: "line"
+					},
+					axis: {
+						y: {
+							min: 0
+						}
+					},
+					grid: {
+						y: {
+							show: true,
+							ticks: 5
+						}
+					}
+				};
+		});
+
+		it("y grid showed with nice intervals?", () => {
+			const yPos = [426, 320, 214, 108, 1];
+
+			chart.$.grid.y.each(function(d, i) {
+				expect(+this.getAttribute("y1")).to.be.equal(yPos[i]);
+			});
+		});
+
 		it("set options", () => {
 			args = {
 				data: {