I\'m trying to build a dataset before plotting it. I decided to use function factory gammaplot.ff() and the first version of my code looks like this:
        If I use rbind or rbind.data.frame, the columns are turned into characters every time. Even if I use stringsAsFactors = FALSE. What worked for me was using
rbind.data.frame(df, data.frame(ColNam = data, Col2 = data), stringsAsFactors = FALSE)
You can use rbind.data.frame and cbind.data.frame instead of rbind and cbind. 
I want to put @mtelesha 's comment to the front.
Use stringsAsFactors = FALSE in cbind or cbind.data.frame:
x <- data.frame(a = letters[1:5], b = 1:5)
y <- cbind(x, c = LETTERS[1:5])
class(y$c)
## "factor"
y <- cbind.data.frame(x, c = LETTERS[1:5])
class(y$c)
## "factor"
y <- cbind(x, c = LETTERS[1:5], stringsAsFactors = FALSE)
class(y$c)
## "character"
y <- cbind.data.frame(x, c = LETTERS[1:5], stringsAsFactors = FALSE)
class(y$c)
## "character"
UPDATE (May 5, 2020):
As of R version 4.0.0, R uses a stringsAsFactors = FALSE default in calls to data.frame() and read.table().  
https://developer.r-project.org/Blog/public/2020/02/16/stringsasfactors/