Pre-calculate box plot statistics

I have a rather large comparative dataset and I'd like to know if there's a way that I can calculate the boxplot summary in advance so that it's not always done at runtime. The idea being to provide the boxplot directly with summary data instead of having pygal calculate the statistics on load.

No files, tests, or entry points are named. Start by locating pygal's box plot implementation and the path that calculates statistics, then determine how summary data could be accepted directly; done means a documented input path avoids recalculating statistics at load time and has coverage for the new behavior.

Written by the indexing model from the issue text.