This repo contains files for a Shiny app that makes maps of California census tracts showing fractions of households paying more than 50% of their income on rent.
Currently the calculations are incorrect (they're households/population) but it's probably a reasonable first step to compare within/across counties, assuming the number of people per household isn't too strongly correlated with rent burden. (It very well might be.)
See the visualization at http://garretchristensen.shinyapps.io/CAHousingPrices.