Mult-Color Line Plot in R Plotly based on Y Value

This was an interesting one. But after a while I realized you can get what you want by inserting a zero value at every zero crossing of your plot:

I think the code is self-explanatory (with the comments)

Here is the code - (with some faked data):

library(plotly)

#fake up some data
set.seed(123)
n <- 100
sdate <- as.Date("2014-03-06")
dt <- seq.Date(sdate,by="days",length.out=n)
results <- data.frame(dates=dt,v1=rnorm(n,32.6,0.2),v2=rnorm(n,32.6,0.2))
results$spread <- results[,3] - results[,2]


# find all the zero crossings
spd <- results$spread
lagspd <- c(spd[1],spd[1:(length(spd)-1)])
crs <- sign(spd)!=sign(lagspd)
results$crs <- crs

# now insert a zero row where there is a crossing
insertZeroRow <- function(df,i){ 
  n <- nrow(df)
  ndf1 <- df[1:i,] # note these overlap by 1
  ndf2 <- df[i:n,] # that is the row we insert
  ndf1$spread[i] <- 0
  ndf <- rbind(ndf1,ndf2)
}


i <- 1
while(i<nrow(results)){
  if (results$crs[i]){
    results <- insertZeroRow(results,i)
    i <- i+1
  }
  i <- i+1
}

# plot it now

results$neg <- ifelse(results$spread <= 0 , results$spread, NA)
results$pos <- ifelse(results$spread >= 0 , results$spread, NA)

plot_ly(results,
        x = ~dates,
        y = ~pos,
        type = 'scatter',
        mode = 'lines',
        line = list(color = 'green')) %>%
  add_trace(results,
            x = ~dates,
            y = ~neg,
            type = 'scatter',
            mode = 'lines',
            line = list(color = 'red')) %>%
  layout(xaxis = list(title = 'Date'),
         yaxis = list(title = 'Price'))

And here is the result:

Note you could make it better by interpolating the dates and spread value to get the correct x-axis crossing point, but I think it would not make a huge difference in most cases. If you did that you would need a date type that can represent hours of the day too (like as.POSIXct), in order to be able to specify the correct x-axis value.

Update:

Just to clear up any confusion, adding the zero rows are necessary. If you comment out the insertZeroRow call, you get this: