How to plot columns with huge amount of observations from two different datasets?

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It's not really possible to give a good answer without a reprex.See the FAQ: How to do a minimal reproducible example reprex for beginners. It;s going to depend on the type of graph, the number of aesthetics to be applied and the number of data points. In the simple case, with a really large number of observations, sampling might work.

library(ggplot2)
d <- data.frame(
  x = sample(1:10000,replace = TRUE),
  y = sample(1:10000,replace = TRUE))

d |> ggplot(aes(x,y)) + geom_point()



d$x <- sample(d$x,100)
d$y <- sample(d$y,100)
d |> ggplot(aes(x,y)) + geom_point()

Created on 2023-06-21 with reprex v2.0.2