Your graph and your friend's graph will likely not look the same. This is because the way you are changing the concentration of the reactant is different.
In your case, when you increased the volume of water in each cup, you were diluting the solution, which means the concentration of reactant particles (in this case, the antacid) was decreasing as you increased the water volume. The graph would show a downward trend as the concentration of reactants decreases.
On the other hand, your friend's method of adding more antacid tablets to equal volumes of water will increase the concentration of reactant particles. Therefore, their graph would show an upward trend, indicating increased concentration as more antacid tablets dissolve in the same volume of water.
Regarding the extension of your graph: as you continue to decrease the concentration of the reactant particles toward zero, the reaction rate will also decrease. This is due to the fact that in a chemical reaction, reactants must collide with enough energy for a reaction to occur. If the concentration of the reactant approaches zero, the likelihood of reactant particles colliding decreases significantly, which means the reaction rate will slow down considerably. Eventually, if the concentration reaches zero, the reaction will stop entirely because there will no longer be any reactant particles available.