Configuring Heatmap for timeseries data?

I have a timeseries metric from simplejson that can supply hourly data (average electrical power over time). I would like to configure the heatmap to show histogram over daily hours, that is in 24 buckets. My expectation was that this should be possible:

Grafana does the bucketing by going through all time series values. The bucket sizes & intervals will be determined using the Buckets options.

What happens though is that I’m always getting a sort of timeseries chart is shown below.

How does the heatmap need to be configured?