Change `ticks.mode` to `scale.distribution` (#4582)

Fix `ticks.mode` behavior when `ticks.source` is `auto`: the lookup table is now built from the data and not from the ticks, so data (and ticks) are correctly distributed along the scale. Rename the option to `distribution` (more explicit than `mode`) and since this option applies from now on the data, it seems better to have it under `scale` instead `scale.ticks`.