Subplot panel height not consistent

There doesn't seem to be a way to set the total figure size for subplot().
Individual panel height can be specified in the call to plot_ly and (as i understand it) that height is scaled with the number of panels to become the total figure size.

I tried using this by passing to each subplot the individual plotheight multiplied with the number of subplots (nrows) so the total height of the total figure would be plotheight * nrows.
It turns out the heights aren't strictly adhered too.
Comparing the total height of the first two panels you can clearly see the x axes are at different heights:

generate_subplot <- function(nrows, margin = NULL){

  if(is.null(margin)) margin = c(0,0,0,0)

  p <- plotly::plot_ly(mtcars, 
    x = ~cyl, y = ~mpg, 
    color = ~rownames(.data), 
    type = "bar",
    height = 150 * nrows
  ) %>% plotly::layout(
    showlegend = FALSE,
    shapes = list(
      list(
        type = 'rect',
        fillcolor = "gray", 
        line = list(color = "gray"), 
        opacity = 0.05,
        x0 = 0, x1 = 1,
        y0 = 0, y1 = 1,
        xref = 'paper',
        yref = 'paper'
      )
    )
  )
  
  plotly::subplot(
    rep_len(list(p), nrows),
    nrows = nrows,
    margin = margin
  )

}

generate_subplot(2)
generate_subplot(4)
generate_subplot(8)

首先运行提供的 generate_subplot 示例，分别使用 2、4 和 8 行，并比较渲染后图形中的面板高度和 x 轴高度。跟踪 subplot() 如何组合重复的 plot_ly 对象及其高度值；当每个面板都具有所请求的一致高度，且图形总高度按预期缩放时，即表示完成。