If you know the equation of the line that you want to draw, then just define a variable for your prediction:
predictedResponseTime <- 30 * users ^ 2
lines(users, predictedResponseTime)
If the problem is that you want to fit a line, then you need to call a modelling function.
Since loess
is a non-parametric model, is isn't appropriate to use it to make predictions outside of the range of your data.
In this case, a simple (ordinary least squares) linear regression using lm
provides a reasonable fit.
model <- lm(responseTime ~ users)
prediction <- data.frame(users = 1:500)
prediction$responseTime <- predict(model, prediction)
with(prediction, lines(users, responseTime))