Reference system : +X points right and +Y points up

I am simulating a simple compression test (2D), where an elastic square block is sandwiched between a fixed, rigid plate on the top and a movable, rigid plate at the bottom. The bottom plate is moved up (+ve Y direction), and the entire bottom plate has a "Rigid Body" constraint with a "Reference Point" at the center of the plate.

If I plot an XY plot of the reaction force (RF2) of the reference point with time, the reaction force is positive and increases with time. This seems counterintuitive to me : if the bottom plate is exerting a force on the block in the upward direction (+be Y), shouldn't the reaction force exerted by the block on the plate be in the downward direction (-ve Y), and hence be -ve in magnitude (with the magnitude increasing with time)?

Abaqus ​​​​​​​