Multicategory axis support

Hi,

I am trying to use multicategory axes and get very inconsistent behavior. This is what I tested so far:

Bar charts:

I need to manually supply type = "multicategory" to the axis. This is opposed to what I have read for the python api, where this will happen automatically if your traces have arrays. see here

reprex

library(plotly)
two_f <- rbind(
  c("BB+", "BB+", "BB+", "BB", "BB", "BB"),
  c(16, 17, 18, 16, 17, 18)
)
p <- plot_ly() %>% 
  add_bars(x=two_f,y=c(1,2,3,4,5,6)) %>% 
  layout(barmode="relative")
p # no multicategory axis
p_multi <- p %>% layout(xaxis=list(side = "top", type = "multicategory"))
p_multi # multicategory axis on top

Heatmaps:

Thanks to michaelbabyn, I know how to create a multicategory x-axis for a heatmap. However, as opposed to bar charts, making a multicategory y-axis crashes, and trying to move the x-axis on top of the chart also crashes.

reprex

library(plotly)
q <- plot_ly() %>% 
  add_heatmap(
    x=rbind(c('aa', 'aa', 'bb', 'bb', 'cc', 'cc'), c(1,2,3,4,5,6)),
    y=c(1,2,3,4,5,6),
    z = array(seq(1,10, length.out = 36), c(6,6)))
q # this works
q_y <- plot_ly() %>% 
  add_heatmap(
    x=c(1,2,3,4,5,6),
    y=rbind(c('aa', 'aa', 'bb', 'bb', 'cc', 'cc'), c(1,2,3,4,5,6)),
    z = array(seq(1,10, length.out = 36), c(6,6))) 
## this would crash the Rstudio session:
## it should render something like https://codepen.io/michaelbabyn/pen/pMOOwM?editors=0010
# q_y

## this also breaks the session:
qx_top <- q %>% layout(xaxis=list(side = "top"))
# qx_top

Start by running the supplied plot_ly(), add_bars(), add_heatmap(), and layout() reproductions in RStudio, comparing the bar and heatmap cases. Trace the multicategory axis handling for x and y axes; done means the examples render consistently, including the heatmap y-axis and top-positioned x-axis, without crashing.