This happens if using a string as a data point, for example in a setup like this:

series: [{
    data: ["3", "5", "1", "6"]
}]

Highcharts expects numerical data values.

The most common reason for this error this is that data is parsed from CSV or from a XML source, and the implementer forgot to run parseFloat on the parsed value.

Note: For performance reasons internal type casting is not performed, and only the first value is checked (since 2.3).