How can I plot a sphere with plotly in R?

Question

I would like to plot a sphere in the following plot:

And that is the code for it:

library(plotly)

fig <- plot_ly(mtcars, x = ~wt, y = ~hp, z = ~qsec,
               marker = list(color = ~mpg, colorscale = c('#FFE1A1', '#683531'), showscale = TRUE))
fig <- fig %>% add_markers()
fig <- fig %>% layout(scene = list(xaxis = list(title = 'Weight'),
                                   yaxis = list(title = 'Gross horsepower'),
                                   zaxis = list(title = '1/4 mile time')),
                      annotations = list(
                        x = 1.13,
                        y = 1.05,
                        text = 'Miles/(US) gallon',
                        xref = 'paper',
                        yref = 'paper',
                        showarrow = FALSE
                      )) %>%   add_markers(x = 2.8, y = 120, z = 20, color="red", marker=list(size=30,
                                                                                                opacity = .65,
                                                                                                line=list(width=2,
                                                                                                          color='black')))
fig

At the moment my best version only contains a 2D circle. How could I integrate a 3d/wireframe sphere into it using plotly R-version?

Answer 1

A sphere is an isosurface. You can plot an isosurface with plotly but it is a bit slow, I prefer to use the misc3d package.

library(plotly)

f <- function(x, y, z){
  x^2 + y^2 + z^2
}
R <- 2 # radius
x <- y <- z <- seq(-R, R, length.out = 100) 
g <- expand.grid(x = x, y = y, z = z)
voxel <- array(with(g, f(x, y, z)), dim = c(100, 100, 100))

library(misc3d)
cont <- computeContour3d(voxel, level = R^2, x = x, y = y, z = z)
idx <- matrix(0:(nrow(cont)-1), ncol=3, byrow=TRUE)

plot_ly(
  x = cont[, 1], y = cont[, 2], z = cont[, 3],
  i = idx[, 1], j = idx[, 2], k = idx[, 3],
  type = "mesh3d"
) %>% layout(scene = list(aspectmode = "data"))