Diagonal annotations in Plotly's categorical heatmap - r

Question

Consider the following R snippet to render a heatmap with categorical axes:
library(plotly)
x <- c("Blue", "Red")
y <- c("One", "Two")
plot_ly(x = x, y = y, z = c(1:4)) %>%
add_heatmap(
opacity = 0.9
) %>%
add_annotations(
text = c(1:4),
showarrow = FALSE
)
This renders the following heatmap:
The annotations appear to be distributed diagonally and unevenly, starting from the bottom left cell. 1 and 3 are in the bottom left cell, and 2 and 4 in the upper right. Why is this? How should my annotation text be structured for it to be ordered more intuitively (horizontally or vertically)?

Answer 1

I can only speculate about the problem but in the provided image you can see that Plotly only used two values out of the 4 z-values. Your colorscale on the right goes from 1 to 2, not 1 to 4. This happens mMost likely because you provided only two x and y values.
Use a data frame
df <- expand.grid(x, y)
df <- transform(df, text = paste(Var1, Var2, sep='_'))
print(df)
Var1 Var2 text
1 Blue One Blue_One
2 Red One Red_One
3 Blue Two Blue_Two
4 Red Two Red_Two
You can now easily use add_annotations
add_annotations(x = df$Var1,
y = df$Var2,
text = df$text)
To get the following plot
Complete code
library(plotly)
x <- c("Blue", "Red")
y <- c("One", "Two")
df <- expand.grid(x, y)
df <- transform(df, text = paste(Var1, Var2, sep='_'))
p <- plot_ly(x = df$Var1,
y = df$Var2,
z = c(1:4)
) %>%
add_heatmap(opacity = 0.9
) %>%
add_annotations(x = df$Var1,
y = df$Var2,
text = df$text)
p
Alternatively you could loop over your values and add an annotation for each one.
library(plotly)
x <- c("Blue", "Red")
y <- c("One", "Two")
p <- plot_ly(x = x,
y = y,
z = c(1:4)
) %>%
add_heatmap(opacity = 0.9)
for (val_x in x)
{
for (val_y in y)
{
p <- add_annotations(p,
x = val_x,
y = val_y,
text = paste(val_x, val_y, sep = '_'))
}
}
p