R plotly Sankey diagram with annotated columns - r

Question

I'm trying to create an annotated Sankey diagram. I'd like the final version to look along the lines of this manually annotated diagram:
The easy part of getting the Sankey diagram:
sankey_diagram <- plot_ly(
type = "sankey",
orientation = "h",
node = list(
label = c("Node_A_1", "Node_A_2", "Node_B_2", "Node_B_3", "Node_C_1", "Node_C_2"),
color = c("blue", "blue", "blue", "blue", "blue", "blue"),
pad = 15,
thickness = 35,
line = list(
color = "black",
width = 0.5
)
),
link = list(
source = c(0,1,0,2,3,3),
target = c(2,3,3,4,4,5),
value = c(8,4,2,8,4,2)
)
) %>%
layout(
font = list(
size = 15
)
)
At first I thought that if I want to get annotated "columns" I should turn to the annotations section of the plotly's documentation. The problem with annotations is that they are spatially limited to (at least I think so) the area of the graph. This is the code in the annotation-based approach:
# properties that hide the axes
ax <- list(
title = "",
zeroline = FALSE,
showline = FALSE,
showticklabels = FALSE,
showgrid = FALSE
)
sankey_diagram %>%
add_annotations(
x=-1,
y=-5,
xref = "x",
yref = "y",
text = "Column A",
xanchor = 'right',
showarrow = F
) %>%
add_annotations(
x=0,
y=-5,
xref = "x",
yref = "y",
text = "Column B",
xanchor = 'right',
showarrow = F
) %>%
add_annotations(
x=1,
y=-5,
xref = "x",
yref = "y",
text = "Column C",
xanchor = 'right',
showarrow = F
) %>% add_annotations(
x=1,
y=1,
xref = "x",
yref = "y",
text = "",
xanchor = 'right',
showarrow = F
) %>%
layout(xaxis = ax, yaxis = ax)
The problem is that annotations land on the bottom of the graph but not under it.
The second approach is based on subplots. I create two subplots - the first one is Sankey, the other one is an empty one except for the annotations - and put them one in a row:
columns_plot <- plot_ly() %>% add_annotations(
x=-1,
y=-5,
xref = "x",
yref = "y",
text = "Column A",
xanchor = 'right',
showarrow = F
) %>%
add_annotations(
x=0,
y=-5,
xref = "x",
yref = "y",
text = "Column B",
xanchor = 'right',
showarrow = F
) %>%
add_annotations(
x=1,
y=-5,
xref = "x",
yref = "y",
text = "Column C",
xanchor = 'right',
showarrow = F
) %>% add_annotations(
x=1,
y=1,
xref = "x",
yref = "y",
text = "",
xanchor = 'right',
showarrow = F
) %>%
layout(xaxis = ax, yaxis = ax)
p <- subplot(sankey_diagram, columns_plot, nrows = 2, shareX = TRUE, margin = 0.1)
p %>%
layout(xaxis = ax, yaxis = ax)
For some strange reason plotly puts the columns_plot on top of the sankey_diagram. I suspect that the second approach is the correct one but still I can't get the result depicted in the first paragraph of this question.