R: Set Plotly hovermode to "compare data on hover"

Question

I am trying to create a stacked area chart in R using the Plotly package as part of a Shiny app and would like to compare the data on hover. However, I am hiding the mode bar for design reasons, so I need to declare this option in my code as currently the hover is only shown for the closest data point to the cursor.

However, the Plotly for R reference only gives the options "x" (tooltip on the x-axis), "y" (tooltip on the y axis), "closest" (shows the tooltip for the closest data point to the cursor) and FALSE (disables the tooltip).

Is there a way to do what I would like? Note that this question is pretty much the complete opposite of this one.

The code I'm using is:

plot_ly(data2, 
        x = ~Year,
        y = ~B, 
        name = 'In-centre', 
        type = 'scatter', 
        mode = 'none', 
        fill = 'tozeroy', 
        fillcolor = '#F5FF8D', 
        hoverinfo = 'y') %>%
add_trace(y = ~A, 
          name = 'At home', 
          fillcolor = '#50CB86', 
          hoverinfo = 'y') %>% 
layout(xaxis = list(title = "", 
                    showgrid = FALSE, 
                    tickangle = 270, 
                    dtick = 1, 
                    tickfont = list(size = 11)),
       yaxis = list(title = "", 
                    ticklen = 8, 
                    tickcolor = "#EEEEEE", 
                    range = c(-2, 101), 
                    tick0 = 0, 
                    dtick = 10, 
                    tickfont = list(size = 11)),
       showlegend = TRUE,
       legend = list(x = 0, 
                     y = -0.2, 
                     orientation = "h", 
                     traceorder = "normal"),
        margin = list(t = 25, b = 50, r = 10, l = 40)) %>% 
config(displayModeBar = FALSE)

where a (simplified version of) data2 is:

Year   A      B
2006   18.0   82.0
2007   19.2   78.3
2008   17.9   80.2
2009   20.1   77.7

Answer 1

Add layout(hovermode = 'compare') to your code:

data2 <- read.table(text="
Year   A      B
2006   18.0   82.0
2007   19.2   78.3
2008   17.9   80.2
2009   20.1   77.7
", header=T)

library(plotly)
library(dplyr)
plot_ly(data2, 
        x = ~Year,
        y = ~B, 
        name = 'In-centre', 
        type = 'scatter', 
        mode = 'none', 
        fill = 'tozeroy', 
        fillcolor = '#F5FF8D', 
        hoverinfo = 'y') %>%
add_trace(y = ~A, 
          name = 'At home', 
          fillcolor = '#50CB86', 
          hoverinfo = 'y') %>% 
layout(xaxis = list(title = "", 
                    showgrid = FALSE, 
                    tickangle = 270, 
                    dtick = 1, 
                    tickfont = list(size = 11)),
       yaxis = list(title = "", 
                    ticklen = 8, 
                    tickcolor = "#EEEEEE", 
                    range = c(-2, 101), 
                    tick0 = 0, 
                    dtick = 10, 
                    tickfont = list(size = 11)),
       showlegend = TRUE,
       legend = list(x = 0, 
                     y = -0.2, 
                     orientation = "h", 
                     traceorder = "normal"),
        margin = list(t = 25, b = 50, r = 10, l = 40)) %>% 
config(displayModeBar = FALSE) %>%
layout(hovermode = 'compare')

EDIT @OctavianCorlade sent me an important note about the solution given above: "The previously provided answer works, simply because any string different than the available options would produce the same result. hovermode = 'x' is the documented way to do it, achieving the exact same result".
Hence, according to the suggestion of @OctavianCorlade, one can use:

layout(hovermode = 'x')