I can show multiple entities in a single history graph card, if they all have the same physical quantity.
However, I often have a case where I want to view multiple entities, having in total two different physical quantities and I cannot view them on the same card, making it harder to view the correlation between the entities. An example might be the output of a heater (e.g. in W or in %) and corresponding temperatures (in degrees C).
The card could automatically decide which physical quantity to assign to which Y-axis, or maybe (the entity of) one physical quantity could be explicitly assigned to an Y-axis using the card configuration.
It would probably be even possible to add two physical quantities and an additional binary state to a single graph by placing the two physical quantities on the two Y-axis, and display the additional binary state entity by a background color.