Preserve milliseconds with dynamicTicks (#1871)

* Fix #1870 by explicitly including milliseconds when converting to JSON

* Added visual test for #1870

* Fix POSIXt format for plotly JSON

According to plotly documentation (https://plotly.com/chart-studio-help/date-format-and-time-series/): "Chart Studio’s date format is 'yyyy-mm-dd HH:MM:SS.ssssss'.", so time_format in jsonlite::toJSON() changed to match.

* Fixing time format in test re: #1871

* Update test image for line-milliseconds.svg

Author: FlukeAndFeather

R/utils.R

@@ -1021,7 +1021,8 @@ try_file <- function(f, what) {
 # preferred defaults for toJSON mapping
 to_JSON <- function(x, ...) {
   jsonlite::toJSON(x, digits = 50, auto_unbox = TRUE, force = TRUE,
-                   null = "null", na = "null", ...)
+                   null = "null", na = "null", 
+                   time_format = "%Y-%m-%d %H:%M:%OS6",  ...)
 }
 
 # preferred defaults for toJSON mapping

tests/figs/lines/line-milliseconds.svg

@@ -41,7 +41,19 @@ test_that("different colored lines become different colored traces", {
   expect_identical(info$data[[2]]$x[1:n], x)
 })
 
-
+test_that("Milliseconds are preserved with dynamic ticks", {
+  d <- data.frame(
+    t = as.POSIXct("1970-01-01 00:00") + (0:999) / 10,
+    y = sin((0:999) * 4 * pi / 1000)
+  )
+  gg <- ggplot(d, aes(t, y)) + geom_line()
+  p <- ggplotly(gg, dynamicTicks = TRUE)
+  j <- plotly_json(p, jsonedit = FALSE)
+  t2 <- jsonlite::fromJSON(j)$data$x[[1]] %>% 
+    as.POSIXct(format = "%Y-%m-%d %H:%M:%OS")
+  expect_equal(as.numeric(mean(diff(t2))), 0.1)
+  expect_doppelganger_built(p, "line-milliseconds")
+})
 
 test_that("Translates both dates and datetimes (with dynamic ticks) correctly", {