Feature: Margin setting

[camdendotlol]

There should be an option to set a margin independently of the gap setting. It would add pixels around the edges of the chart.

This would require:

• a new attribute in the Vuex state object
• a line in migrations.ts to migrate existing charts to a default value
• an update to the topster package to draw the margins
• an update to the interactivity code to account for the margins when calculating position