Rename parameter 'svg' to 'group'
Remove intermediate 'g' element wrapping each tile
Highlight author circles on hover

The circle is highlighted when the mouse cursor hovers either
over the tile circle or the adjacent tile bounding box.

The hover effect is a bit slow in Firefox, not as responsive
as in Safari. I will check whether applying the effect with
a descendent selector when a grouping tile is hovered makes
any difference.
Wrap each tile in a 'g' element to capture 'hover' for CSS styles

The performance is visibly better in Firefox, while still using
the adjacent sibling selector, with a number of adjacent siblings
limited to 1 within the 'g' element of each tile.
Simplify selector for hover effect on each tile

I replace two selectors, including an uncommon adjacent sibling
selector, with a single child selector.
Move bounding box from bounding box 'rect' back to tile 'g'