Makefile with two C++ files : How can I choose the file to compile?

Let's say I have two C++ main files, project.cpp, and projectgraph.cpp. They both use the same header file functions.h, as well as the file functions.cpp.

To compile project.cpp, I'm using the following makefile :

CXX = g++
CXXFLAGS= -std=c++11 -w -Wall -g 

project: project.o functions.o 
    $(CXX) $(CXXFLAGS) -o project projet.o functions.o 

functions.o: functions.h

clean:
    rm -rf *.o

I would like to be able to choose between project.cpp and projectgraph.cpp to be compiled using the make command in the terminal. For example :

• If I type make in the terminal : project.cpp would be compiled.
• If I type make graph in the terminal : projectgraph.cpp would be compiled.

How can I change the makefile to get this result ?

Thanks !