Added unit test for issue #4921.

samples/issues/highcharts-4.2.1/4921-ignorehiddenseries-columnwidth/demo.details

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+---
+ authors:
+   - Øystein Moseng
+ resources:
+   - https://code.jquery.com/qunit/qunit-1.19.0.js
+   - https://code.jquery.com/qunit/qunit-1.19.0.css
+...

samples/issues/highcharts-4.2.1/4921-ignorehiddenseries-columnwidth/demo.html

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+<script src="https://code.highcharts.com/highcharts.js"></script>
+
+<div id="qunit"></div>
+<div id="qunit-fixture"></div>
+
+
+<div id="container" style="width: 600px; margin: 0 auto;"></div>

samples/issues/highcharts-4.2.1/4921-ignorehiddenseries-columnwidth/demo.js

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+$(function () {
+    QUnit.test('Check that column width takes hidden series into account', function (assert) {
+        var chart = Highcharts.chart('container', {
+                chart: {
+                    type: 'column',
+                    ignoreHiddenSeries: false
+                },
+                series: [{
+                    name: 'Tokyo',
+                    data: [49.9, 71.5, 106.4, 129.2, 144.0, 176.0, 135.6, 148.5, 216.4, 194.1, 95.6, 54.4]
+                }, {
+                    name: 'Tokyo2',
+                    data: [49.9, 71.5, 106.4, 129.2, 144.0, 176.0, 135.6, 148.5, 216.4, 194.1, 95.6, 54.4]
+                }]
+            }),
+            colWidth = chart.series[0].points[0].pointWidth;
+
+        chart.series[0].hide();
+        chart.series[1].hide();
+        chart.series[0].show();
+
+        assert.strictEqual(
+            chart.series[0].points[0].pointWidth,
+            colWidth,
+            'Columns remain same width as with 2 series visible'
+        );
+    });
+});