$YC2YR is the yield spread between 10-year US Treasuries and the 2-yr. $SPX is the S&P 500.
Both are shown as percentage change over 20 years in Figure 1. In Figure 2, the inverse relationship between the yield curve spread and the direction of the S&P 500 is marked with indicative trend lines. In Figure 3, the yield spread value (right scale) and the S&P 500 price level (left scale) are superimposed.
Figure 1:
Figure 2:
Figure 3: