Pivot on single column, but with diagonal historic customer visit view

Question

With data that is reflected as: df = pd.DataFrame({\'CustID\': [1,2,3, 2,4,5,1, 6,5,2,7,3,8,9,5,4], \'YearVisited\': [2013,2013, 2014, 2014, 2015, 2018, 2019, 2019, 2019, 2020,