Equivalent of abline in plotly

Question

I would like to add a line (not just vertical or horizontal) to a plotly chart

library(plotly)
d = data.frame(x=1:10, y=1:10)
plot_ly(d, x = ~x, y = ~y, type='scatter')

Say I want (D) y = 2x as a line is there a way for me to plot without generating the data myself in another column of d

Answer 1

There at least two ways of simulating abline.

• Add another trace with the start and end point
• Add a line shape to the layout with the start and end point

The disadvantage is that the min and max for each trace needs to be calculated (Plotly needs to do that anyway but it bloats the code).

The alternative would be draw the line from one extreme to the other other and then set the xaxis and yaxis range manually.

In the example below the red line is the added trace, while the black line is added via shape line.

library('plotly')

#a function to calculate your abline
p_abline <- function(x, a=2, b=-5){
  y <- a * x + b
  return(y)
}

#create an empty plot    
p <- plot_ly()

#list with all your x and y values
x <-  list(c(1,2,3), c(2,3,6), (1:5))
y <-  list(c(1,2,3), c(0,5,10), sample(-10:10, 5))

#loop through the x/y values and determine min/max
for (i in 1:length(x)) {
  p<-add_trace(p, y=y[[i]], x=x[[i]] , type="scatter", mode="markers+lines")
  if (i == 1 || max(x[[i]]) > max_x) {
    max_x = max(x[[i]])
  }
  if (i == 1 || max(y[[i]]) > max_y) {
    max_y = max(y[[i]])
  }
  if (i == 1 || min(x[[i]]) < min_x) {
    min_x = min(x[[i]])
  }
  if (i == 1 || min(y[[i]]) < min_y) {
    min_y = min(y[[i]])
  }

}

#add a trace to simulate abline
p<-add_trace(p, x=c(min_x, max_x), y=c(p_abline(min_x), p_abline(max_x)) , type="scatter", mode="lines", name='abline')

#add a shape (line) to simulate abline
p <- p %>% layout(
  shapes=list(type='line', x0=min_x, x1=max_x, y0=p_abline(min_x, -1, 3), y1=p_abline(max_x, -1, 3))
  )

p

Answer 2

library(plotly)
d = data.frame(x=1:10, y=1:10)
plot_ly(d, x = ~x, y = ~y, type='scatter') %>%
  add_trace(d, x = ~x, y = ~x * 2, type = "scatter")

It turns out you can do math right within the add_trace function call where you're saying what y should be. So you can tell plotly to just multiply that column by two right in the argument.