I think we should define two values for each dimension here (so repeat this for width as well):

// non-negotiable - this is the smallest height we will allow users to specify via explicit margins
var MIN_SPECIFIED_HEIGHT = 2;

// could be exposed as an option - the smallest we will allow automargin to shrink a larger plot
var MIN_REDUCED_HEIGHT = 64;

Then down below we can combine these two:

var minFinalHeight = Lib.constrain(
    height - margin.t - margin.b,
    MIN_SPECIFIED_HEIGHT,
    MIN_REDUCED_HEIGHT
);

The goal is:

• if you specify a plot height larger than MIN_REDUCED_HEIGHT, automargin is allowed to increase the top and bottom margins to reduce the plot height as low as MIN_REDUCED_HEIGHT.
• if you specify a plot area smaller than MIN_REDUCED_HEIGHT but larger than MIN_SPECIFIED_HEIGHT, automargin will make no changes.
• if you specify a plot height smaller than MIN_SPECIFIED_HEIGHT, automargin should reduce the margins symmetrically to increase the plot height back to MIN_SPECIFIED_HEIGHT