I'm dealing with a quantity that diminishes over time from 100% to 0%. I'm trying to plot the values, a lm abline, and large indicative points where the graph intersects
y==0%. I'm finding the results of
lm(y~x) don't appear to be the inverse of
lm(x~y) and I'm confused why this should be for a deterministic calculation. This illustrates in
set.seed(100) d = data.frame(t = sort(sample(1:60,10)), q = sort(sample(1:100,10), dec=T)) # linear model - dependant y fit = lm(q ~ t, d) # predicting x where y == c(100,0) - i.e. dependant x pred = predict.lm(lm(t ~ q, d), data.frame(q=c(100,0)), se.fit=T)$fit pred = data.frame(t = pred, q = c(100,0)) plot.new() plot.window(xlim=c(0,60), ylim=c(0,100)) axis(1); axis(2) points(d) abline(fit) points(pred, pch=16, cex=2, col='red')
Why don't the red points not fall on the line? I know the modelling/plotting here is probably topsy-turvy, but I'm interested in the principal..