Formatting Math Symbols and Expressions in ggplot Labels

Yesterday, I was trying to put some finishing touches on a figure I made in ggplot2 that visualizes some simulation results. The plot features several panels using facet_grid(), and uses colors to distinguish between different regression models that were fit to the simulated data. I wanted to label certain axes and panel names using the Greek letters I had used as parameter notation, and I also wanted the labels in the color legend to correspond to the different regression models I had fit.