R Plotly stacked bar chart issue

Question

I am using plotly to create a stacked bar chart something like below

df <- data.frame(
  var1 = c("NY", "NY", "NY", "SFO", "SFO", "SFO"),
  var2 =  c("Low", "Medium", "High", "Low", "Medium", "High"),
  value = c(10,15,20,10,20,25)
)
df$var2 <- factor(df$var2, levels = c("Low","Medium","High"))

# sort the factor table
df <- arrange(df, var2)

col <- list(
  color = ifelse(
    df$var2 == "Low",
    "steelblue",
    ifelse(
      df$var2 == "Medium",
      "darkorange",
      "olivedrab"
    )
  ))

# Plot
plot_ly(
  df,
  x = ~ var1,
  y = ~ value,
  color = ~ var2,
  marker = col,
  type = "bar"
) %>% layout(barmode = "stack")

There are 2 issues I am facing: First is that the legend is not coming correctly (see image).

Second is that since what I have is a dynamic table, there is a possibility that there is only one value in var1 and var2 so it will be something like

df <- data.frame(var1 = c("NY"),
                 var2 =  c("Low"),
                 value = c(10))
df$var2 <- factor(df$var2, levels = c("Low", "Medium", "High"))

# sort the factor table
df <- arrange(df, var2)

col <- list(color = ifelse(
  df$var2 == "Low",
  "steelblue",
  ifelse(df$var2 == "Medium",
         "darkorange",
         "olivedrab")
))

# Plot
plot_ly(
  df,
  x = ~ var1,
  y = ~ value,
  color = ~ var2,
  marker = col,
  type = "bar"
) %>% layout(barmode = "stack")

in which case the chart doesn't show up. If I have 2 values either in var1 or in var2 the chart still shows up but not with one. Can someone tell me how to handle this?

thanks, Manoj