How to get the coordinate of the nodes of a circular tree by ggtree

[ARise-fox]

I want to know how to get the node coordinates of the deformed ggtree. The coordinates obtained by ggtree::fortify() are valid in a square tree, but I don't know how to get coordinates for other tree points with shapes such as circular or slanted. I guess it's going to take some polar conversion. Here is my code to make tree:

library(ggtree)
tr <- rtree(10)
ggtree::fortify(tr)
ggtree(tr,layout = "fan")->a

The fortified data is not consistent with the plane cartesian coordinates of the points on the fan-shaped graph.

# A tibble: 19 × 9
   parent  node branch.length label isTip     x     y branch angle
    <int> <int>         <dbl> <chr> <lgl> <dbl> <dbl>  <dbl> <dbl>
 1     11     1        0.614  t6    TRUE  0.614  1     0.307   36 
 2     13     2        0.659  t9    TRUE  1.83   2     1.50    72 
 3     14     3        0.647  t5    TRUE  2.21   3     1.89   108 
 4     15     4        0.0817 t2    TRUE  2.62   4     2.58   144 

I tried to parse the ggplot object directly, using ggplot2::ggplot_build() Here is my code:

ggplot2::ggplot_build(a)[["data"]][[2]]

The output is, take node t6 (node 1 , parent11) as example:

$data[[2]]
   node parent         x        y PANEL group      xend      yend colour linewidth linetype alpha
1     1     11 0.0000000 2.953125     1    -1 0.0000000  1.000000  black       0.5        1    NA

But t6 is definitely not anywhere near (0,2.95) or (0,1). The expected coorinate is more like the middle of the plot. I really need help, please.