  We study the two-dimensional border-collision normal form (a four-parameter
family of continuous, piecewise-linear maps on $\mathbb{R}^2$) in the robust
chaos parameter region of [S. Banerjee, J.A. Yorke, C. Grebogi, Robust Chaos,
Phys. Rev. Lett. 80(14):3049--3052, 1998]. We use renormalisation to partition
this region by the number of connected components of a chaotic Milnor
attractor. This reveals previously undescribed bifurcation structure in a
succinct way.
