Because this widget can display any "single" value (regardless of whether it's a number or not), a row with multiple fields in it would not make sense as you would have multiple potential values to pick from; therefore, if there are multiple rows, or there are multiple fields in a row, the widget will present an error.

An exception to the above behaviour is possible by using the timeChart() function, in which case there can be multiple rows or data, used to calculate a trend — see Displaying a Trend with a Timechart for more details.

The Single Value widget is often used in conjunction with functions such as sum(), count(), or avg() which produce a single row with a single field, like _sum.

The widget can also be used on data where you have produced a single row with only one field. The select() and selectLast() functions are very useful for this.

For single-value results based on grouping on a single field, the widget also supports data visualization in multiple charts arranged in a grid — one small chart for each of the field values — allowing them to be easily compared. See Displaying Values in a Grid.