Fixed #6897, prevented axis extremes change in case of a single point…

… if multiple ticks are set.

js/parts/Axis.js

@@ -2248,8 +2248,9 @@ H.extend(Axis.prototype, /** @lends Highcharts.Axis.prototype */{
 		this.trimTicks(tickPositions, startOnTick, endOnTick);
 		if (!this.isLinked) {
 
-			// Substract half a unit (#2619, #2846, #2515, #3390)
-			if (this.single) {
+			// Substract half a unit (#2619, #2846, #2515, #3390),
+			// but not in case of multiple ticks (#6897)
+			if (this.single && tickPositions.length < 2) {
 				this.min -= 0.5;
 				this.max += 0.5;
 			}

samples/unit-tests/axis/ticks/demo.details

@@ -0,0 +1,6 @@
+---
+ resources:
+   - https://code.jquery.com/qunit/qunit-2.0.1.js
+   - https://code.jquery.com/qunit/qunit-2.0.1.css
+ js_wrap: b
+...

samples/unit-tests/axis/ticks/demo.html

@@ -0,0 +1,7 @@
+<script src="https://code.highcharts.com/stock/highstock.js"></script>
+
+
+<div id="qunit"></div>
+<div id="qunit-fixture"></div>
+
+<div id="container" style="width: 600px; height: 250px; margin: 0 auto"></div>

samples/unit-tests/axis/ticks/demo.js

@@ -0,0 +1,30 @@
+QUnit.test('Ticks for a single point.', function (assert) {
+    var chart = Highcharts.chart('container', {
+        yAxis: {
+            tickPositioner: function () {
+                return [0, 0.2, 0.4, 0.6, 0.8];
+            }
+        },
+        series: [{
+            data: [0.2]
+        }]
+    });
+
+    assert.strictEqual(
+        chart.yAxis[0].min,
+        0,
+        'multiple ticks from tickPositioner for a single point (#6897)'
+    );
+
+    chart.yAxis[0].update({
+        tickPositioner: function () {
+            return;
+        }
+    });
+
+    assert.strictEqual(
+        chart.yAxis[0].min,
+        -0.3,
+        'single tick and increased extremes for a single point'
+    );
+});