ggplot图例的颜色与辅助变量。

I'd like to create a plot with a quadratic curve in black, and horizontal lines at various levels in different colours, with a legend (guide) that labels the horizontal line colours. I can't quite figure it out -- I've tried a couple of variations that seem to make sense to me, but I can't seem to force the legend to appear.I'd like to create a plot with a quadratic curv