[Bar] Round single side

[techniq]

visx has a BarRounded component that does this, leveraging a <path> instead of a <rect>. I've debated on whether this should be a separate component, or if LayerChart's would switch between these underlying elements based on the props used.

Pros of single component:

• Easier to use (just pass different props)

Cons of single component:

• How to handle tweening the width/height when using <path> (what <Rect> provides)?

[techniq]

Resolved via #169