Hello Joachim,
One way is to define an attribute that has the formula
correlation(attribute1,attribute2)
as shown in the attached screenshot. Notice that by dragging the new attribute all the way to the left to promote it, you get a single value that is less confusing than if you left it in the “flat” part of the table.
Another way is to make a scatterplot of the two attributes and, with the ruler icon on its inspector, add a least squares regression line. On the line you will see the value of r-squared.
Bill Finzer