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Select a spreadsheet from the drop down list below or from the standard control in the lower left corner
of the right-hand side of the Gantt chart.<p><p>When a spreadsheet is active certain other features (e.g. zoom in / out)
are not available anymore.<p><p>To edit a cell, simply click in it and start typing. Different spreadsheets support
different data types: numbers, strings, enums.
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