A contour trace draws a scalar field as curves of constant
value. Plotly computes those curves while it draws and keeps none of
them — its calculated data holds the grid and nothing else — so MAIDR
walks the grid itself, by marching squares, at the levels the trace
states in contours. Each curve becomes one navigable row:
the level is announced on the field's own axis rather than as a series
name, and every point announces the distance to the nearest point on the
adjacent level, which is what the crowding of the lines says visually.
Nothing has to be declared. contour and
histogram2dcontour name themselves, and plotly resolves the
level ladder even when autocontour chose it.