JSON export? Vega export? Use Vega.jl? Use Deneb.jl?

Instead of directly creating an SVG plot with Gadfly, the app could just export the downsampled data as JSON, which could then, for example, be included from a Vega spec. Or generate Vega JSON directly. Or use Vega.jl. Or Deneb.jl?

Start by reviewing how the app currently creates SVG plots with Gadfly and where the downsampled data is produced. The issue presents several unselected approaches—JSON export, Vega JSON, Vega.jl, or Deneb.jl—so first establish the desired format and library; done should be a documented choice with a working export path.

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