As said, as the concentration of atropine increased, the dose-response curve for acetylcholine moved further to the right. The extent of this shift to the right is a measure of how potent the antagonist is: the stronger the antagonist, the greater the shift to the right for a given antagonist concentration.
Antagonist A Antagonist B
The same antagonist concentrations have been used. Antagonist A is a more potent antagonist than B.