Multisubject Annotations & Custom Point Marks

Features:

• All axes will automatically draw a <line> element with class axis-baseline along the baseline of the axis. You can restyle or hide it with CSS.
• react-annotation annotations in XYFrame and ORFrame will honor a coordinates array and draw multi-subject annotations when you send datapoints in that array.
• customPointMark in XYFrame can now return any JSX SVG instead of just a semiotic-mark <Mark> element. It also passes { d, i, xScale, yScale } so you can draw marks whose shape depends on data values in chartspace.

Fixes:

• motifs mode in NetworkFrame is working again. Singletons are now sorted into a single component.

You can see most of this in action in the Comet Plots example.