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Matrix is a two dimensional array, with inner arrays corresponding to each column's contents.  Columns need not have the same number of entries.    The lists within the matrix may not contain nulls..

Each widget may have configuration associated with it.  It will be legal for configuration to be absent, in which case, defaults will apply.  Configuration will be divided into configuration that may be associated with any widget (for instance, refreshPeriod or state), and configuration that associates with widgets of particular types (for instance, limit).  The widget configuration's is opaque to the dashboard serialisation format. The widget configuration does not encapsulate the widget's type.  To discover the type, a further REST API call will be necessary.

To illustrate the the format further, if I wanted a format dashboard like this:

    id2
id1 id3
    id4

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