y = functions[2];
	const y2s = functions[3];
	const x0 = s2x(w / 2);
	const y0 = s2y(h / 2);
	var y = 0.0;
	// function
	const f_text = document.getElementById("f").value;
	const f_lines = f_text.split('\n');
	for(var line_i = 0; line_i < f_lines.length; ++line_i) {
		const f_line = f_lines[line_i];
		try {
			eval("var f = function(x) { return " + f_line + ";}")
			document.getElementById("error").innerHTML = "";
			if(abs(f(x0) - y0) < abs(y - y0))
				y = f(x0);
		} catch (error) {
			var f = function(x)