Each element can be either an array ['column', 'format'] or a shortcut string
'column:format'.
Column can be a string representing the attribute, field, or key from the
source data. It can even be an integer if the source data uses numeric keys.
Format can be one of the built-in formatters ('datetime', 'float', 'int', 'raw', 'string', or
'timestamp') or a callable that is used to prepare each data value. If format is omitted, the
raw value will be passed to the chart.
Example showing different ways to specify a column:
php
[
['date_measured', 'datetime'],
'open',
'high:float',
['low', 'float'],
['close', function($value) {
return ceil($value);
}]
]
