Q:

Highcharts DataGrouping with Null values

Having an issue where we're trying to show a gap in data when nulls are present in the data set with dataGrouping enabled on the Highstock chart.
If the data set is small, like in the images below, leaving dataGrouping disabled isn't an issue. 
The problem comes in when we have a very large data set, 2 days worth of data x 4 series = 690,000+ data points; the data takes forever to load.
If we leave dataGrouping enabled, then the gap in the data disappears.
Is it possible to get keep dataGrouping enabled but also show the gap for nulls?

A:

The problem is caused by your data - all of the null values are strings:
200: (2) [1569876448000, "null"]
201: (2) [1569876449000, "null"]
202: (2) [1569876450000, "null"]
203: (2) [1569876451000, "null"]
204: (2) [1569876452000, "null"]
205: (2) [1569876453000, "null"]
206: (2) [1569876454000, "null"]
207: (2) [1569876455000, "null"]
208: (2) [1569876456000, "null"]
209: (2) [1569876457000, "null"]
210: (2) [1569876458000, "null"]
211: (2) [1569876459000, "null"]

As a solution you should change the format or parse the data in the complete function:
data: {
    csvURL: '...',
    complete: function(options) {
        options.series.forEach(function(s) {
            s.data.forEach(function(p) {
                if (p[1] === 'null') {
                    p[1] = null;
                }
            });
        });
    },
    enablePolling: false
}

Live demo: https://jsfiddle.net/BlackLabel/c0xd5j62/
API Reference: https://api.highcharts.com/highcharts/data.complete

