Issue with NA handling in scatter plots: Two NAs per category cause incorrect line connection

Issue Summary

When using plot_ly() in R with a scatter plot (mode = "lines+markers"), missing (NA) values are expected to create gaps in the line plot. However, if exactly two NA values exist per category, the missing values are incorrectly connected by a line instead of creating a gap.

Interestingly, when the hovertemplate is removed, the line plot behaves as expected (i.e., creating a gap for NA values). This issue only occurs when there are exactly two NA values per category; the code works with any other number of NA values.

Additional Discovery:
The issue is resolved if I include the argument split = ~Category, but I cannot find documentation for split in Plotly, which makes me think it may be deprecated. Moreover, when the hovertemplate is removed, the inclusion of split does not work as expected and does not resolve the issue.

Reproducible Example

The following R code demonstrates the issue:

library(plotly)

df <- data.frame(
  Category = rep(c("A", "B"), each = 6),
  Date = c(2020, 2021, 2022, 2023, 2024, 2025, 2020, 2021, 2022, 2023, 2024, 2025),
  Value = c(10, 15, NA, NA, 20, 25, 12, 14, NA, 22, NA, 27)
)

df$Date <- factor(df$Date, levels = unique(df$Date), ordered = TRUE)

plot_ly(
  df,
  x = ~Date,
  y = ~Value,
  color = ~Category,
  type = 'scatter',
  mode = 'lines+markers',
  text = ~Category,
  hovertemplate = paste0("Date: %{x}<br>Category: %{text}")
)

Expected Behaviour

• NA values should create a gap in the line plot, i.e., they should not be connected.
• This works correctly when there is any number of NA values other than exactly two in any category.

Actual Behavior

• When there are exactly two NA values per category, the missing values are incorrectly connected by a line instead of creating a gap.
• When the hovertemplate is removed, the lines create a gap as expected.
• The issue only arises when there are exactly two NA values per category; any other instance of NA works fine.
• Including split = ~Category resolves the issue, but:
  • I cannot find any Plotly documentation on split, leading me to believe it may be deprecated.
  • Interestingly, when the hovertemplate is removed, including split does not resolve the issue.

Additional Notes
This seems to be an issue specifically triggered by the combination of NA handling and the hovertemplate. I would appreciate further insight on why this happens or suggestions for a workaround to preserve the gap in the case of two NA values.

System Info

R version 4.4.2 (2024-10-31 ucrt)
Platform: x86_64-w64-mingw32/x64
Running under: Windows Server 2019 x64 (build 19045)

Matrix products: default

まず、issue にある再現可能な plot_ly() の例を実行し、hovertemplate と split = ~Category がある場合とない場合で出力を比較します。R の plotly 呼び出しが NA 値とカテゴリのトレースをどのように表現するかを追跡し、その後、カテゴリごとにちょうど 2 つの NA があれば、split を必要とせず、hovertemplate を削除しなくても目に見える空白が生じることを確認します。