Heartbeat charts can be created directly from the Heartbeat menu, or when navigating the results of an Explore comparison.
Directly from the Heartbeat menu
Select 'Heartbeat' from the platform menu then click on the 'Charts' tab
Click the blue plus button in the top right and complete the required fields, then click 'Create Heartbeat chart'
Themes can also be defined and Heartbeat charts created when navigating the results of a comparison in Explore.
Drag and drop the things you want to track into the theme builder side panel
Click 'Create Heartbeat chart' to apply those themes to the data associated with your current comparison.