[thclark]

Automatic contour levels are now spaced logarithmically when a contour's colour scale has a logarithmic norm. The fix ships with the bars and histograms release (0.16.0), so this PR targets the epic trunk. The patch bump to 0.15.1 that it carried for a release from main has been dropped.

Before and after

Before, contour_levels in crates/ironlab-scene/src/compile/artists.rs always placed linear levels. A field spanning several decades therefore had all its isolines in its top decade. In the issue's reproduction, the levels were 100, 200, …, 900.

Under a logarithmic norm, Levels::Auto, and a contour with no levels of its own on a continuous scale, now choose levels strictly inside the range of the field's positive values. They are chosen in this order:

1. the decades, thinned to at most the requested count;
2. if fewer than two decades lie inside the range, 1, 2 and 5 times each decade;
3. failing that, 1 to 9 times each decade;
4. failing that, linear levels.

The reproduction now draws isolines at 0.1, 1, 10 and 100.

Band colours

A filled band under a logarithmic norm now takes the colour of its geometric middle, rather than its arithmetic middle, so that it matches its place on the colourbar.

Unchanged and documented

A discrete scale's band edges are unchanged. The contour section of the figure schema reference documents the new levels.

Closes #167