Date tickformats automatically replaces newline \n with <br>. Causing the same value on the second line to repeat.

Custom date tickformat does not respect the newline character \n and automatically replaces it by a break-tag <br />. This has the consequence that values after the specified newline get repeated for each tick-label even though the value is the same.

As this python plotly documentation mentions:

Date axis tick labels have the special property that any portion after the first instance of '\n' in tickformat will appear on a second line only once per unique value, as with the year numbers in the example below. To have the year number appear on every tick label, '<br>' should be used instead of '\n'.

Ploty python and javascript respect the \n but not R.

Minimal example:

library(plotly)
#> loading comments I have committed for this reprex
dt <- data.frame(
    x = as.Date(c('2013-02-04', '2013-04-05', '2013-06-06', '2013-08-07', '2013-10-02')),
    y = c(1,4,5,6,2)
  )
plot_ly(data = dt, x = ~x, y = ~y, type = 'scatter', mode = 'lines') %>%
  layout(xaxis = list(tickformat = '%a %e %b \n %Y'))

^{Created on 2022-03-31 by the reprex package (v2.0.1)}

In specting the above object with plotly_json() and search for tickformat reveals that the format has been updated to: %a %e %b <br /> %Y.

This reprex is an example taken from the javascript documentation where the example shows the correct behaviour of only showing the year 2013 once.

Start with the R minimal example in issue #2115 and inspect its output with plotly_json(), focusing on how the x-axis tickformat converts \n. Compare the result with the linked JavaScript and Python behavior; done means R preserves \n so repeated date values are displayed according to the documented behavior.