subplot() doesn't account for dual axes

I think there is something wrong with the subplot function when there is yaxis2

I tried to use subplot with 2 plots that have yaxis2. Each plot looks like this:

foo_plot <- function(n) {
  x <- rnorm(n)
  den <- density(x)
  p <- plot_ly()
  p <- add_trace(p, x=x, type="histogram", name="Histogram",
    marker=list(line=list(color="#000", width=1), color="#555"))
  p <- add_trace(p, x=den$x, y=den$y, yaxis="y2", name="Density",
    type="scatter", mode="lines", line=list(color="#222"))
  p <- layout(p,
    yaxis2=list(overlaying="y", side="right", rangemode="nonnegative",
      showgrid=F, showline=T, ticks="outside"),
    yaxis=list(showline=T, ticks="outside"))
  return(p)
}

p <- subplot(foo_plot(1000), foo_plot(1000), nrows=2, shareX=F, shareY=F)

The density curves were missing when I merge the two plots.

This is the result of sessionInfo()

首先运行 issue 的 foo_plot 和 subplot 示例，将每个绘图与合并后的两行结果进行比较。跟踪 subplot 对 yaxis2 的处理，并验证合并后两条密度曲线仍然可见；完成标准是双轴轨迹在合并后的绘图中能够正确渲染。