fix(zoom): Correct zoom in rendering on 0 coord.

When mouse over the coordinates after zooming in, the guide line has
incorrect coordinates.

Fix #169
Close #170

spec/api.zoom-spec.js

@@ -45,6 +45,23 @@ describe("API zoom", function() {
 			expect(Math.round(domain[0])).to.be.equal(target[0]);
 			expect(Math.round(domain[1])).to.be.equal(target[1]);
 		});
+
+		it("should be zoomed and showing focus grid properly when target contained minus value", () => {
+			const target = [-2, 3]; // zoom in cotaining minus value
+
+			chart.zoom(target);
+
+			const zoomScale = chart.internal.zoomScale;
+
+			// If target contained minus value should not be null
+			expect(zoomScale).to.not.be.null;
+
+			const domain = chart.internal.zoomScale.domain();
+
+			// domain value must be above than target
+			expect(Math.round(domain[0])).to.be.above(target[0]);
+			expect(Math.round(domain[1])).to.be.above(target[1]);
+		});
 	});
 
 	describe("zoom line chart #2", () => {

src/interactions/zoom.js

@@ -103,14 +103,17 @@ extend(ChartInternal.prototype, {
 		// 	.on("dblclick.zoom", null);
 
 		if ($$.zoomScale) {
-			const range1 = $$.zoomScale.domain()[0];
-			const range2 = $$.x.domain()[0];
-			const delta = 0.015;
-
-			// reset scale when zoom is out as initial
-			if (range1 <= range2 || (range1 - delta) <= range2) {
-				$$.zoomScale = null;
+			const zoomDomain = $$.zoomScale.domain();
+			const xDomain = $$.x.domain();
+			const delta = 0.015; // arbitrary value
+
+			// check if the zoomed chart is fully shown, then reset scale when zoom is out as initial
+			if (
+				(zoomDomain[0] <= xDomain[0] || (zoomDomain[0] - delta) <= xDomain[0]) &&
+				(xDomain[1] <= zoomDomain[1] || xDomain[1] <= (zoomDomain[1] - delta))
+			) {
 				$$.xAxis.scale($$.x);
+				$$.zoomScale = null;
 			}
 		}