savings-rate.scm calcualtes net-profit as a percentage of income i.e the savings rate.
As the percentage is negligible compared to income/expenses, it won't be visible in the charts.
The table below the chart would report savings rate, but it is tagged with the currency value of the report rather than a % symbol. I don't have much knowledge on the gnucash utilities to convert it to a plain number.
Tested with Gnucash v5.4