The diff complexity icon, shown in the diff viewer's file index and in

the Review Request Updated boxes, has historically used jquery-flot to

do the rendering. This rendered to a canvas, had a white stroke area,

and did not scale well. It was also not accessible, and not easy to

style. It was a non-starter for dark mode.
This change rewrites this entirely to output SVGs. We do this by hand,

as bringing in a library like D3 at this stage would take some care as

to not impact Power Pack.
The code generates the segments, normalizes percentages to enforce

minimums, and then generates SVG path data for each of the segment arcs.

It calculates an outer radius and an inner radius, as we did with flot,

and give us a graph that correctly shows an overview of the changes made

to the file.
It also comes with a new tooltip and accessibility information

describing the number of changes to a file, and what types of changes

were made. All the relevant numbers are included.
Styling is done with CSS, giving us full control over light and dark

mode styling.

Tested a wide range of boundary conditions for the icon in Storybook,

and compared to the original implementation in each (using a flag I

had in development for triggering the original rendering logic).
Compared against the old rendering for a number of test diffs. Aside

from very minor differences (new color palette, very slight rendering

changes), the appearance was the same.
Tested in light and dark modes.
Checked the accessibility tree.