Display color legend when data has only one category

Question

I just found out that if a variable is mapped to the color argument in plotly, the legend is automatically displayed only if the variable has more than one category. This is illustrated below. df1 has two categories and they are displayed in the legend . df2, on the other hand, has only one category and no legend is displayed. My goal is to display the legend in the df2 plot.

library(plotly)

df1 <- data.frame(
  x = 1:100,
  y = rnorm(100),
  category = sample(c("cat", "dog"), 100, replace = TRUE)
)

plot_ly(
  type = "scatter",
  mode = "markers",
  data = df1,
  x = ~x,
  y = ~y,
  color = ~category
)

f2 <- data.frame(
   x = 1:100,
   y = rnorm(100),
   category = "dog"
)

plot_ly(
  type = "scatter",
  mode = "markers",
  data = df2,
  x = ~x,
  y = ~y,
  color = ~category
)

Answer 1

You can add a legend with the argument showlegend:

plot_ly(
  type = "scatter",
  mode = "markers",
  data = df2,
  x = ~x,
  y = ~y,
  color = ~category
) %>% layout(showlegend = TRUE)