If the ability to conduct independent research is the most valuable skill for a PhD... A master graduate could close that gap by publishing several papers during his/her career? Maybe he/she can also become more valuable since the additional years in the job market (as /u/dvorakcoder said)

I thought that when comparing master degree vs PhD the latter wins because it can ask for a better salary, or am I wrong?

What is the x-axis? I am assuming that are the months just because there are twelve different cells... Just write the axis guys