What does the "O" line represent on the chart? What is indicated by a sharp spike below the “O” line?

A sharp spike below the 0 line means to go further than you think you need to go

Explanation: this is what google said

(202 bc - 220 ad) dynasty started by lui bang; a great and long-lasting rule, it discarded the harsh policies of the qin dynasty and adopted confucian principles; han rulers chose officials who passed the civil service exams rather than birth; it was a time of prosperity

"i care deeply about campaign finance reforms: - apex