This is a much larger problem than it indicates, and isn't just a wage gap issue.
For as bad as HASS (humanities, arts, social science) are to major in, the graph itself makes HASS look better than they are already, and this is common to most graphs comparing HASS to STEM: HASS makes something like 40k, while STEM makes 70k. But that 40k is not right after college, while for STEM it might be, because HASS income might be from skills acquired post-college while STEM income tends to be pegged to skills acquired in-college. A lot of the data that is given to college majors is usually to prove a college degree pays itself off over time. And it does, but not in the way people think.
If you start at some entry-level clerk position, and over time you're promoted to a higher position that requires a degree (or considered because you have a degree), yes, the return on that degree will be higher, but the degree was able to be a requirement to begin with because so many people have them now, and the average does not take into account the effect of current debt levels on starting wages. Putting something like 45k annual income as the average for English majors (which I think Discover does, and they give out loans iirc) is setting students up for disastrous post-college debt, especially when private loans set the repayment terms and IBR is unavailable for those loans, because the repayment terms can be as much as a mandatory $500 - $1000/month independent of cost of living and many of these students can expect $400-500/week for their first job after college. The economic effects of this could be worse than you think.
Starting wage is extremely important, and starting wage is far more certain with STEM majors in addition to simply better.
I suspect that we're looking at something much worse than a wage gap, since a lot of students are taking on huge debt to pay for majors that seem like sound investments given the data they provided, but in fact are not. We need data that specifies annual income 12, 24, and 36 months after graduation for college majors, specific to each college; otherwise, how are students supposed to make informed decisions? Annual averages for the general population tell you nothing, especially for HASS majors. Better data would elucidate both the wage gap issue and the debt issues we're facing.
A way to obtain this data that I doubt anyone is going to actually enforce would be to require colleges to report better information if they're to receive suchandsuch federal benefit. First, though, we need to actually want better data.