Minor release supporting Vega releases 5.8 and 5.9.

Additions

New transforms

trTimeUnit transform and associated time binning parameterisation functions tbUnits, tbStep, tbTimezone, tzLocal, tzUtc, tzSignal, tbInterval, tbExtent, dtMillis, dtExpr, tbMaxBins, tbSignal, and tbAs.
quarter and date added as time units.
trKde2d two-dimensional (raster) density estimation transform and its associated parameterisation functions kd2Weight, kd2CellSize, kd2Bandwidth, kd2Counts and kd2As.
trIsocontour transform and associated property functions icField, icThresholds, icLevels, icNice, icResolve, icZero, icSmooth, icScale icTranslate and icAs for more flexible contouring.
trHeatmap transform and associated property functions hmField, hmColor, hmOpacity, hmResolve and hmAs
trFlattenWithIndex and trFlattenWithIndexAs for nested array flattening that output the array index of flattened data.

axTickBand for aligning ticks on band scales (Vega 5.8).
axTranslate for moving axes relative to main plot area (Vega 5.8).
axFormatAsTemporalUtc for UTC formatting (Vega 5.8).
leFormatAsTemporalUtc for UTC formatting (Vega 5.8).
bnInterval for specifying whether or not both bin boundaries are output in bin transform (Vega 5.8).
maCornerRadiusTopLeft, maCornerRadiusTopRight, maCornerRadiusBottomLeft and maCornerRadiusBottomRight for use with rect and group marks (Vega 5.8).
maScaleX and maScaleY to path mark properties (maAngle for path marks now rotates path although no changes to API) (Vega 5.8).
maSmooth added to image mark properties (Vega 5.8).
maImage for dynamically created image marks added to image mark properties (Vega 5.8).
maStrokeForeground for overlaying group stroke over content (Vega 5.9).
maStrokeOffset for shifting group stroke and fill (Vega 5.9).
mollweide map projection (Vega 5.9).
inLabels for providing input element labels that may differ from their option values (Vega 5.9).

trContour and its associated cn property functions are now deprecated in favour of trIsocontour (Vega 5.8)