This data is scraped from Indeed. The area you choose is within a 25KM radius and goes through 30 pages of Job postings (400+ Jobs).
The accuracy is quite high. The reason for this is each job posting is analysed thoroughly.
The only issue is some job postings do list multiple programming languages/technologies
so it can't always be completely accurate.
If a technology is the highest on the chart, this means it was the most mentioned across all job postings in your area.