
Internal helper: read a series' aesthetics out of a style_spec
Source:R/style_internals.R
series_aes.RdLayers the builders set inline rather than tagging with
ggstylekit::series_layer() (VPC ribbons, the LLOQ line, and the
plot_vpc_legend() proxies) source their aesthetics from the style_spec's
per-series maps keyed by series through this reader.