Thanks for the feedback! I'll try to label the chart a little more clearly. The y axis is expected returns, the x axis is expected riskiness. I show the generated optimal portfolios alongside the original constituent assets that comprise the portfolios. So ideally you want a portfolio up and to the left, but higher-returning securities tend to be riskier.
I found the literature on vine copulas to be helpful, particularly the following presentation: https://www.birs.ca/workshops/2013/13w5146/files/Brechmann.p...
I used that as a helpful guide for ways to create some of the more interesting returns simulations.