Data compression

The data option of a series supports three basic ways of of providing data. However, Highcharts requires x values on a datetime axis to be given in JavaScript timestamps, which are milliseconds since 1970-01-01.

For a large time based dataset, passing individual x values over the network may represent a substantial amount of traffic. For example, a dataset with hourly values over a year (24 * 365 = 8760 x/y pairs), weighs 158 kB on the file system. It should be noted though that gzipping would chip off most of the repetetive parts of the timestamp in a typical web setup.

data: [

Regular data

For regularly spaced data points, we don't need the X values at all. All we need to provide is the pointStart and pointInterval options, and the X values will be computed from there. Supplemented with a pointIntervalUnit, the X data will also handle the irregular time intervals that emerge in a local timezone when crossing over the Daylight Saving Time boundaries. Our data set is now down to 18 kB.

data: [1, 1, 1, 1, 1, 1, ...]
pointStart: 1577836800000, // Date.UTC(2020, 0, 1)
pointInterval: 36e5, // one hour

View live demo

Irregular data

But those simplified settings won't work when the X values are irregular. Since v9.1.2, the relativeXValue option is available. This allows us to redefine what the x value means, so that it is first multiplied by pointInterval (and respecting pointIntervalUnit), then pointStart is added to it. As a result, we can compress any X values by the f(x) = ax + b formula where a = pointInterval and b = pointStart. Non-datetime X values can also be compressed this way. Our data set is now 78 kB, but can express irregularly spaced X values.

data: [
pointStart: 1577836800000, // Date.UTC(2020, 0, 1)
pointInterval: 36e5, // one hour
relativeXValue: true

View live demo of relativeXValue