chartlet 0.7.1

Fixes only, from a render sweep across sizes, languages and variants and three code reviews of
0.7.0. Specifications that rendered with 0.7.0 still render, except a declared axis step that
would give more than 50 ticks (or none), which is now refused instead of drawing an unreadable or
endless axis.

Fixed

• A time axis counts the ticks a step actually places instead of estimating them, and spaces its
  ticks by the width its plot actually has: a narrow chart no longer ends up with a single tick,
  and labels with a time of day get the room they need.
• A low pane of a time chart labels only the value ticks that stand a label's height apart; its
  gridlines stay.
• The value label of a short negative horizontal bar no longer runs into the category labels;
  its category leaves out its value labels instead (value_labels_omitted).
• PNG output warns with glyph_missing about characters its fonts have no glyph for.
• A declared valueAxis.step must give at most 50 ticks (an exact axis at least 2), and a
  declared timeAxis.step 1 to 50; a tiny step could draw millions and hang the renderer.
  Stacked bar charts now use a declared valueAxis.step, which they ignored.
• Tooltips and the description write a value axis unit after the values.
• A plot keeps at least 40 pixels of height and reports dense_chart instead of turning upside
  down on a small chart; a calendar reports cells smaller than 4 pixels.
• The value-axis gutter of time charts grows past 72 pixels where long labels, such as a value
  with its unit, need it (up to 40 % of the chart); small multiples measure their panel gutter
  the same way, report labels that do not fit, and shorten or leave out a unit that has no room.
• Zoom windows show their own span: timeAxis.min, max and step apply to the whole axis
  only, and stepEnd only to a window that holds the last observation.
• stepEnd after trailing missing values no longer draws over the gap.
• A declared coarser timeAxis.precision no longer removes ticks within a day.
• An unnamed layer's data table column carries the value axis unit.
• The legend of range bar groups wraps instead of running off a narrow chart.
• A sparkline refuses valueAxis.unit, valueAxis.step and timeAxis.step, which need axes.
• A PNG of a chart without a mobile variant fails with missing_mobile, like the SVG.
• labels_thinned names /data for charts with data, and its message reads "one category in
  n".