Legend size for planetary map / bubble chart

Question

Here is the plot “bubble” map (that is, a map with markers on it, the size of which maps to a variable). However, the legend only displays color categories, but it does not show how the size relates to the value.

library(plotly)

DF = data.frame(
  Group = c("A",  "B",  "A",  "B", "A", "C", "C"), 
  Value = c(100,  80,   90,  150, 120,  60, 110), 
  lat =  c( 40,   32,   36,   44,  31,  39,  37), 
  long = c(-90, -100, -120, -110, -90, -80,-105))


plot_geo(DF, locationmode = 'USA-states') %>%
  add_markers(y=~lat, x=~long, color=~Group, size=~Value, 
    marker=list(sizeref=0.1, sizemode="area")) %>%
  layout(geo=list(scope = 'usa'))

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Answer 1

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legend.sizes = seq(80, 160, 20)
ax = list(zeroline = FALSE, showline = FALSE, showticklabels = FALSE, showgrid = FALSE)
mk = list(sizeref=0.1, sizemode="area")

p.map = plot_geo(DF, locationmode = 'USA-states') %>%
  add_markers(x = ~long, y = ~lat, color = ~Group, size = ~Value, marker = mk) %>%
  layout(geo = list(scope = 'usa'))

p.legend = plot_ly() %>%
  add_markers(x = 1, y = legend.sizes, size = legend.sizes, showlegend = F, marker = mk) %>%
  layout(xaxis = ax, yaxis = list(showgrid = FALSE))

subplot(p.legend, p.map, widths = c(0.1, 0.9))