Residuals from 1:1 line

Question

I want to measure the distance between a set of points and a 1:1 line. I can build a linear model and get the residuals from the best fit, but I cant get the measure from a 1:1 line. Any helpful hints?

#build a df of random numbers     
x=runif(100, 0, 100)
    y=runif (100, 0, 100)
    df=cbind (x,y)
    df=as.data.frame(df)
#build a linear model    
lm1<-lm(y~x, data=df)
    summary (lm1)
#plot the data, lm best fit and 1:1 (red) line)    
    plot (y~x, data=df, pch=16)
    line (lm1)
    abline abline(0,1, col="red")
#get residulas for the linear model 
y.resid= resid (lm1)

Answer 1

I suggest using y-x, just like @vpipkt suggested. Just for the sake of completeness: you can also create a linear model with fixed coefficients y-x ~ 0 and take the residual there.

resid(lm(y-x ~ 0))

Of course this is just more complicated and gives the same result as y-x, but it explicitely states that you are taking residuals and not calculating the minimal distance to the line (cf @user3969377's answer).