chartlet 0.3.0

Fixes and additions from moving a large site to 0.2.0. The specification keeps
schemaVersion: 1; every specification that rendered with 0.2.0 still renders, with the changes
in output listed under Changed and Fixed.

Added

• Time charts: precision per data layer, and every layer writes its times as finely as its
  own observations need — yearly means beside monthly values say only the year, and on a
  numeric axis a finely spaced helper line no longer adds decimals to the other layers. A table
  row is labelled as finely as the finest layer with a value there.
• Time charts: markers: false on a line or area layer, such as a fitted line; its tooltips
  stay on invisible targets.
• A stacked pane takes up to eight area layers.

Changed

• An exact value axis puts its ticks on multiples of the step, with the zero line:
  −2.5 to 3.5 with step 1 ticks −2, −1, 0 … 3.

Fixed

• A horizontal value axis labels only the ticks whose labels fit, so the wide labels of a
  logarithmic axis no longer run into each other.
• A horizontal range bar chart measures its value labels with the fallback reserve, so the
  label at the right end is no longer cut off.