Fix #2462: dynamicTicks now works with grouped geom_line

NA values in trace data (e.g., from geom_line gaps between groups) now
correctly remain as NA when mapping categorical data back to ticktext
labels. Previously, which.min() on NA values returned an empty vector
which caused an indexing error.

Fixes #2462

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cpsievert

R/ggplotly.R

@@ -843,10 +843,12 @@ gg2list <- function(p, width = NULL, height = NULL,
 
       # inverse transform categorical data based on tickvals/ticktext
       if (isDiscreteType) {
-        traces <- lapply(traces, function(tr) { 
+        traces <- lapply(traces, function(tr) {
           # map x/y trace data back to the 'closest' ticktext label
           # http://r.789695.n4.nabble.com/check-for-nearest-value-in-a-vector-td4369339.html
           tr[[xy]]<- vapply(tr[[xy]], function(val) {
+            # NA values (e.g., geom_line gaps) should remain NA
+            if (is.na(val)) return(NA_character_)
             with(axisObj, ticktext[[which.min(abs(tickvals - val))]])
           }, character(1))
           tr