Stat over 2 db tables

How do I show a graph that spans multiple database tables?

Right now I have a graph with 2 lines on it (we’ll call that A & B values), and we manually use a calculator to add A + B for a given time period to get the sum of both lines / db tables.

How do I get a graph with a single line on it, which is the value of A+B for all time periods?

Or do I have to create a new database table with my A+B values in it and just use Metabase to visualize that?