Add horizontal line to barplot in plotly R including legend

Question

I would like to create a barplot with a horizontal line in plotly R. There is an option to use add_segments like here. Unfortunately it doesn't display the full width of the graph and cuts of at the centers of the bars. Here is a simple reproducible example:

library(plotly)
library(dplyr)

iris %>%
  group_by(Species) %>%
  summarise(sepal_length_tot = sum(Sepal.Length)) %>%
  plot_ly() %>%
  add_trace(mode = "bars", x = ~Species, y = ~sepal_length_tot, color = "Species") %>%
  add_segments(x = "setosa", xend = "virginica", y = 200, yend = 200, color = "line")

^{Created on 2025-08-22 with reprex v2.1.0}

As you can see it is not using the full width of the graph. So I was wondering how we can add a horizontal line to a barplot in plotly R and display it in the legend?

This might help... plotly.com/r/horizontal-vertical-shapes

Andrew Gustar
– Andrew Gustar

2025-08-22 09:33:28 +00:00
Commented Aug 22 at 9:33 — Andrew Gustar
– Andrew Gustar, Commented Aug 22 at 9:33

lailaps · Accepted Answer · 2025-08-23 10:13:51Z

4

The problem occurs, because plotly uses a categorical x-axis for the 3 bars, thus adding a line trace has to follow the 3 category values. Therefore, the line can not extend over the xaxis factor values. It becomes more clear, if you limit the bar widths and look into plotly_json

You could just add symbols / annotations but they will not appear as trace and thus have no legend (e.g. add layout(annotations = list(x = -0.5, y = 200,showarrow = TRUE, arrowhead = 1,ax = 1100, ay = 0))).

What works however is to use a second continues xaxis xaxis2 and overlay it on top of the categorical one. Then we add a trace on the second axis and it will appear within the legend and cover the whole plot width. I used the trick explained here

library(plotly)
library(dplyr)

iris |>
  summarise(sepal_length_tot = sum(Sepal.Length), .by = Species) |>
  plot_ly() |>
  add_trace(type = "bar", x = ~Species, y = ~sepal_length_tot, color = "Species")|>
  layout(xaxis2 = list(overlaying='x', range=c(0,2), showticklabels=F))  |>
  add_trace(type = "scatter", mode = "lines", x = c(0, 2), y = c(200,200), color = "line", xaxis='x2', showlegend=T)

edited Aug 23 at 10:13

answered Aug 22 at 8:46

lailaps

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3 Comments

M-- Aug 22 at 16:39

This is truly smart; however, I don't enjoy maintaining/debugging plots with dual-axis, specially when dealing with subplots and whatnot. On a separate note, while mode = "bars" works, you should use type = "bar" (I understand that comes from the original post).

lailaps Aug 22 at 16:56

I think it's a good idea here to use a 2ndary x-axis if OP wants to simply add a threshold across the whole x-axis with, it is simpler to do. But if they want to add multiple threshold lines which align with the boxplots, then your approach will be simpler to use. It's up to the usecase really

M-- Aug 22 at 17:08

Agreed. As I said, this is really smart. I was just sharing my experience/frustration dealing with dual-axis plots. I have lots of them; and to plotly's credit, at least they're not limited to transformations like ggplot is (yeah, I understand Hadley's argument there, but ...).

M-- · Accepted Answer · 2025-08-22 16:30:41Z

We can use a numeric x-axis:

library(dplyr)
library(plotly)

iris %>%
  group_by(Species) %>%
  summarise(sepal_length_tot = sum(Sepal.Length)) %>%
  mutate(x_numeric = row_number() - 1) %>% 
  plot_ly() %>%
  add_trace(type = "bar", x = ~x_numeric, y = ~sepal_length_tot, 
            name = "Sepal Length", color = "Species", 
            colors = "#8da0cb") %>%
  add_segments(x = -0.5, xend = 2.5, y = 200, yend = 200, 
               line = list(color = "#66c2a5", width = 2), name = "Threshold", 
               hovertemplate = "Threshold: %{y}<extra></extra>") %>%
  layout(xaxis = list(tickmode = "array", title = "Species",
                      tickvals = ~x_numeric, ticktext = ~Species))

^{Created on 2025-08-22 with reprex v2.1.1}

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