Toggle fill on hover using plotly with Shiny in R - r

Question

I would like to create a plotly plot where the fill under each line is toggled upon mouseover/hover. The closest that I've come is using a combination of plotly and Shiny in the code below. Basically, I use the function event_data("plotly_hover") with a call to add_trace, which generates the fill for the line. However, when the mouse is moved away from the line, or unhovered, I get an error message: Error: incorrect length (0), expecting: 2366. In addition, the hoverinfo text no longer appears, or only briefly before the fill appears.
I'm not sure what the program is looking for when unhovering, so am not sure why I'm getting this error. Or perhaps there is different and simpler way to toggle the fill for plotly graphs?
ui.R
shinyUI(fluidPage(
titlePanel("Snow Weather Stations"),
mainPanel(
plotlyOutput("testplot", height = "500px")
)
)
)
server.R
library(shiny)
library(plotly)
library(dplyr)
library(tidyr)
csvf <- read.csv(file = "http://bcrfc.env.gov.bc.ca/data/asp/realtime/data/SW.csv",
check.names = FALSE, stringsAsFactors = FALSE)
swe <- csvf %>%
gather(STATION, SWE, -1) %>%
separate(`DATE (UTC)`, c('DATE', 'TIME'), sep = " ") %>%
filter(TIME == "15:00:00") %>%
select(-TIME) %>%
filter(substr(STATION,1,2) == "1A")
swe$DATE <- as.Date(swe$DATE)
swe$HOVERTEXT <- paste(swe$STATION, paste0(swe$SWE, " mm"), sep = "<br>")
xmin <- as.numeric(as.Date("2015-10-01")) * 24 * 60 * 60 * 1000
xmax <- as.numeric(as.Date("2016-09-30")) * 24 * 60 * 60 * 1000
shinyServer(function(input, output) {
output$testplot <- renderPlotly({
plot_ly(swe, x = DATE, y = SWE, group = STATION,
line = list(color = '#CCCCCC'),
text = HOVERTEXT, hoverinfo = "text+x",
hoveron = "points",
key = STATION) %>%
layout(showlegend = FALSE,
hovermode = 'closest',
xaxis = list(title = "",
showgrid = FALSE, showline = TRUE,
mirror = "ticks", ticks = "inside", tickformat = "%b",
hoverformat = "%b %-d",
range = c(xmin, xmax)),
yaxis = list(title = "Snow Water Equivalent (mm)",
showgrid = FALSE, showline = TRUE,
mirror = "ticks", ticks = "inside",
rangemode = "tozero")) %>%
config(displayModeBar = FALSE)
d <- event_data("plotly_hover")
if (is.null(d)) {
stn <- "1A01P Yellowhead Lake"
} else {
stn <- d$key[1]
}
add_trace(filter(swe, STATION == stn), x = DATE, y = SWE,
line = list(color = "404040"), fill = "tozeroy")
})
})