The layout of the tooltip is:
"Column 0 value"
"Data Column Name": "Data Column Value"
You can set the formatted value of a cell to override what the API would normally place there. Ex:
data.setFormattedValue(0, 0 , 'Limousin');
would change 'FR-L' to 'Limousin' in the tooltip. You can use DataTable#addRow() instead of setValue() as a shortcut:
data.addRow([{v: 'FR-L', f: 'Limousin'}, 200]);
which populates the entire row. Pass it an array with values for each column in the row, where each value is either the data type appropriate for that column (string, number, etc) or an object {v: value, f: 'string formatted value'}.