I could not find the results by locality here
https://presidente2018.registraduria.gov.co/resultados/2html/resultados.html Where did you find them?
Results in Bogotá by localidad (borough) present a truly fascinating picture of what happened:
A perfect north-south, rich-poor polarization. Duque won in the wealthiest localities in the north of the city, with 60% in Usaquén (the wealthiest) and 56.5% in Chapinero (the second-wealthiest). He also won more narrowly in Suba, Barrios Unidos and Teusaquillo -- all of which voted for Fajardo in R1 (with Duque a strong second) and which are relatively well off. On the assumption that patterns were similar to the first round, in Suba Duque likely destroyed Petro in the wealthiest neighbourhoods like Niza while Petro won strata 2-3 areas which aren't as wealthy. That being said, Petro pulling 31.8% in Usaquén and 35.1% in Chapinero is still quite notable, as he won less than 15% of the vote in both localities three weeks ago, showing that many, probably most, Fajardo voters even from upper middle-class backgrounds voted for him.
On the other hand, Petro dominated by massive margins in the poor areas in the south of the city, where he had already won over 45% three weeks ago. In Usme, he won 70%, his best result in the city with the obvious exception of Sumapaz (79.8%). He won 68.6% in San Cristobal, 66% in Bosa and 68% in Cd. Bolívar -- the poorest strata 1-2 localities of the city. He also won 64% in Santa Fe, which has both poor areas and more downtown bohemian/student-y areas, 64% in La Candelaria (the small historical core which has only a small resident population, with many young people, students and low-income residents), 63.8% in Rafael Uribe Uribe and 60.8% in Tunjuelito (which both include poorer residential areas).
As I have said in all of my posts analyzing results within cities, elections in urban Colombia are decided by the middle-class strata 3. Fajardo's victory in the first round in Bogotá was in large part due to his very strong numbers in strata 3 localities like Engativá and Fontibón (as well as in Kennedy, the largest locality). In the runoff, Petro easily won in Kennedy - which has a large lower-income strata 2 population as well - with 57%, but also - crucially - defeated Duque by narrower margins in both Fontibón (49.9-43.3) and Engativá (51.1-42.3), where Fajardo won over 40%.
Unlike in the first round and most other elections, the voto en blanco was not highest in lower-income areas, but rather in the affluent north (7.9% in Usaquén and 8.5% in Chapinero) and, unsurprisingly, in other localities where Fajardo had done well.
