The data from GitHub that the chart displays, is based on results from actively polling the GitHub events API. Whenever a new push event is detected, the lines that are changed in each of the commits in the push event are collected and stored.
Results from Stack Overflow are based on the number of times that a tag for a certain language is applied, together with the applied count of the synonyms of that language. This data is refreshed every four hours in order to keep it up-to-date.
The percentage that is shown is the average of the GitHub and StackOverflow ratio for a language.
Please take into a account that the results are based on live recording of the data. In time, when data is collected for other languages besides the ones that are already shown, these languages will automatically pop up in the chart.