As the Frontend Traffic dashboard presently exists, it reports an availability fraction -- just 1 - ((5XX responses) / (all requests)) -- for each site (eqiad, esams, ...) and cache type (text, upload).
That's all fine and good. But then it computes a 'global' availability number simply by adding together all the ratios from each site -- which have wildly different denominators -- and then subtracting 1 from the whole sum. This doesn't produce a meaningful number.
Here's an illustration, with the status quo in thick red and the correct computation in thick green:
https://grafana.wikimedia.org/d/uiBRF5hWk/xxx-cdanis-frontend-traffic-copy?panelId=3&fullscreen&orgId=1&from=1570101355987&to=1570110394094