Why are yields shown per bed, and can I use another bed length?

Because you plan in beds, and a yield per bed is a number you can check against your own harvest. In Crop yield, yields are counted per bed of your standard length.

To read them on another length, click Settings in the chart and change the Bed reference length. Heirloom converts the yields and the revenue per bed. It changes this chart’s display only, not your beds.

The steps are in Set your yields and prices.