this.getBBox is undefined?

this here is an SVG element (because autoBox is passed to svg.attr in the cell above). https://developer.mozilla.org/en-US/docs/Web/API/SVGGraphicsElement/getBBox

Width is undefined here

width is part of the Observable standard library (but you can define a fixed width if you prefer not to have a responsive chart): https://github.com/observablehq/stdlib/blob/master/README.md#width