MongoDB’s BSON document structure can store array data types. In order to interpret that data for Designer, the user has to decide how they want those multi-value fields to be displayed.
Row-wise array expansion takes an array of values and expands them in a single column creating a new row for each value in the array. If there are multiple row-expanded arrays in the same document, then the number of rows generated is equal to the largest array size. Additionally, an Automatic x-axis column is automatically generated for use as an x-axis value for visualizations in Designer using array data.
To use the row-wise array expansion feature, select Row-Expanded Array from the Structure drop-down box.
This feature will only work for an array data type. If the actual data type in MongoDB is not array or the array is empty, the column will not populate.