In July 2020, the Spanish government published an official index of rental prices by getting the data from the Tax Authority, the national cadastre, the Spanish Statistics Office, the property registry, the Bank of Spain and the Spanish regional authorities.
It's amazing how the data needed for such a public index is so much broken up and divided across so many different institutions.
One of the indexes published is the evolution of the average rental price by square meter between 2015 and 2018, which shows an increase of 19% in Malaga and Palma (two touristic cities), 17% in Barcelona, 16% in Valencia and 13% in Madrid.
Using that data, newspapers like El País and elDiario published interactive maps where you can zoom into almost street level to see how the rental prices evolved between 2015 and 2018.
Visualisation by El País here: https://elpais.com/economia/2020-07-03/alquileres-de-ricos-y-pobres-el-mapa-de-precios-calle-a-calle.html
Visualisation by elDiario here: https://www.eldiario.es/economia/subido-alquiler-grandes-ciudades-calle_0_1044296362.html
The Government publishes the data here: https://www.mitma.gob.es/vivienda/alquiler/indice-alquiler