You can use the Heat Map widget to visualize results of the groupBy(), top(), bucket(), and timeChart() query functions. In all cases, the result needs to contain two grouping variables and a third variable with the aggregate result. Thus, for groupBy() and top() you need to specify two fields to group by, and for bucket() and timeChart() you need to specify a single field to group by along with the timestamp.

The Heat Map widget is typically well suited when there is an inherent ordering to both the axis variables and you want to have, for instance, an overview of how many events fall into each combination of those variables.