support "group" property to break lines

"group" property doesn't create any new trace, but if the trace
mode is "lines", NA points are inserted at the "group" boundaries
within the same trace. That is interpreted as the polyline break by
plotlyjs.

Author: alyst

R/plotly.R

@@ -339,14 +339,29 @@ plotly_build.plotly_hash <- function(l = last_plot()) {
       # define the dat traces
       points_df <- data.frame(dat_index = seq_along(dat[["x"]] %||% dat[["y"]] %||% dat[["z"]])) %>% # indices of the original data elements used in the trace FIXME properly define data length
         tracify_by_color(dat) %>%
-        tracify_by_column(dat, "symbol", force_numeric=TRUE)
-      trace_key_cols <- setdiff(colnames(points_df), "dat_index")
-      points_df <- dplyr::arrange_(points_df, .dots = c(trace_key_cols, "dat_index")) %>%
-                   dplyr::group_by_(.dots = trace_key_cols)
+        tracify_by_column(dat, "symbol", force_numeric=TRUE) %>%
+        tracify_by_column(dat, "group", force_numeric=TRUE)
+      subtrace_key_cols <- setdiff(colnames(points_df), "dat_index")
+      trace_key_cols <- setdiff(subtrace_key_cols, "group_index")
+      points_df <- dplyr::arrange_(points_df, .dots = c(subtrace_key_cols, "dat_index")) %>%
+                   dplyr::group_by_(.dots = subtrace_key_cols)
+      points_df$subtrace_index <- dplyr::group_indices(points_df)
+      points_df <- dplyr::group_by_(points_df, .dots = trace_key_cols)
       points_df$trace_index <- dplyr::group_indices(points_df)
       points_df <- dplyr::ungroup(points_df)
       points_df$point_order <- seq_len(nrow(points_df))
 
+      # polylines should be further disrupted at 'group' boundaries by inserting NAs
+      if (grepl("lines", dat[["mode"]] %||% "markers+lines") && "group_index" %in% subtrace_key_cols) {
+        subtrace_bound <- points_df$trace_index[-1] == points_df$trace_index[-nrow(points_df)] &
+                          points_df$subtrace_index[-1] != points_df$subtrace_index[-nrow(points_df)]
+        if (any(subtrace_bound)) {
+          points_df <- rbind(points_df, points_df[subtrace_bound,]) %>% dplyr::arrange(point_order)
+          points_df$dat_index[c(FALSE, points_df$point_order[-1] == points_df$point_order[-nrow(points_df)])] <- NA
+          points_df$point_order <- seq_len(nrow(points_df)) # order wrt added points
+        }
+      }
+
       trace_point_indices <- attr(dplyr::group_by(points_df, trace_index), "indices")
       if (length(trace_point_indices) > 0) {
         trace_point_indices <- lapply(trace_point_indices, function(ixs) ixs+1L)

tests/testthat/test-plotly.R

@@ -25,6 +25,20 @@ test_that("Using group argument doesn't transform the data in markers mode", {
   expect_identical(l$data[[1]]$group, iris$Species)
 })
 
+test_that("Using group argument for scatter plot in lines mode introduces breaks (NA values separating different groups)", {
+  p <- plot_ly(data = iris, x = Sepal.Length, y = Petal.Length, group = Species, mode = "lines")
+  l <- expect_traces(p, 1, "scatterplot-group-lines")
+  expect_identical(l$layout$xaxis$title, "Sepal.Length")
+  expect_identical(l$layout$yaxis$title, "Petal.Length")
+  n_groups <- length(unique(iris$Species))
+  n_pts <- length(l$data[[1]]$x)
+  expect_identical(n_pts, nrow(iris)+n_groups-1L)
+  expect_identical(sum(is.na(l$data[[1]]$x)), n_groups-1L)
+  expect_true(all(is.na(l$data[[1]]$group[-1]) |
+                  is.na(l$data[[1]]$group[-n_pts]) |
+                  (l$data[[1]]$group[-1] == l$data[[1]]$group[-n_pts])))
+})
+
 test_that("Mapping a variable to symbol works", {
   p <- plot_ly(data = iris, x = Sepal.Length, y = Petal.Length, symbol = Species)
   l <- expect_traces(p, 3, "scatterplot-symbol")