How to interpret errors.ubuntu.com graph data?

What does the "frequency" column measure, in what units?

Number of instances of that problem for the selected period.

A instance is one person experiencing a specific error. These errors have signatures which make them unique. A grouping of all of the instances with the same signature is a "problem".

In simpler terms, the frequency is the number of times this specific problem was encountered and reported.

What does the "Mean Time Between Failure" vertical axis on the graph mean?

This has since been replaced with "Average number of crashes." This is the total number of reports seen in the day divided by the number of unique users sending those reports.

What do the "If all updates were installed" vs "Actual" toggle mean? For me, clicking "Actual" just blanks the whole graph.

This is a placeholder. "If all updates were installed" will show the graph only for those users who had completely up to date systems. The gap between this ideal line and the "actual" line tells us the degree to which we need to fix our updates mechanism.

Where is the code that generates the page?

lp:errors