The upper-most purple horizontal line indicates the overnight highs from Friday, as it was a failure point on Friday as well.
A key to the chart above as to what the horizontal lines represent. The Fib. projections are based off of the overnight range high/low:
Something else I usually have on screen is the Advance/Decline difference for the S&P500 (top chart) and the Up/Down Volume difference (bottom chart) for the S&P500. What I'm most interested in is the interaction with the zero line (horizontal teal) and how deeply positive or negative the index is for the day. Volume started negative and trended that way all day. While with the Advance/Decline, the zero-line highlighted a good support-turned-Resistance pivot: