Shapes/images don't accumulate with multiple calls to layout()

line1 <- list(
  type = "line",
  x0 = 0,
  x1 = 1,
  y0 = 0.5,
  y1 = 0.5,
  xref = "paper",
  yref = "paper",
  line = list(color = "#C3C4C6")
)

line2 <- list(
  type = "line",
  x0 = 0,
  x1 = 1,
  y0 = 0.75,
  y1 = 0.75,
  xref = "paper",
  yref = "paper",
  line = list(color = "#C3C4C6")
)

plotly_empty() %>%
  layout(shapes = line1) %>%
  layout(shapes = line2) 

首先运行 issue 中的 R 示例，并确认第二次调用 layout() 会替换第一个形状，而不是累积它。跟踪 plotly.R 包中对 layout() 的处理，然后验证在连续调用之后两个线形状仍然保留。