If an error is throw (because a cover doesn't have a width for example) it would be helpful to see the block or know the block ID.

The API is pretty clunky at the moment, but necessarily so.

For specific graphs, the API could be simplified to something like:

css = rig.passthrough block, config, [{
  selector: '#background'
  query: '(max-width: 503px)'
  width: 400
}]

• specific graphs

For simple media query breakpoints with the same selector, the API could be something like:

css = rig.breakpoints block, config, graph, params,
  selector: '#background'
  breakpoints: [504, 1008]
  widths: [500, 1000, 1200] # or heights

This means that the 500px width image would be used for the media queries like so:

@media (min-width: 503px) { /* 500px wide image */ }
@media (min-width: 504px) and (max-width: 1007px)  { /* 1000px wide image */ }
@media (min-width: 1008px)  { /* 1200px wide image */ }

Any number of breakpoints could be supported as long as breakpoints.length == widths.length + 1.

• breakpoints

This should be doable given that it was done in imgflo/imgflo-url#2.

Basically the same as imgflo/imgflo-url#6, removes the need for 8a6561e.