The `a F` command fits a set of data to a *model formula*,
such as ‘`y = m x + b`’ where ‘`m`’ and ‘`b`’ are parameters
to be determined. For a typical set of measured data there will be
no single ‘`m`’ and ‘`b`’ that exactly fit the data; in this
case, Calc chooses values of the parameters that provide the closest
possible fit. The model formula can be entered in various ways after
the key sequence `a F` is pressed.

If the letter `P` is pressed after `a F` but before the model
description is entered, the data as well as the model formula will be
plotted after the formula is determined. This will be indicated by a
“P” in the minibuffer after the help message.