How do you force my 3D graphing system to graph between -71 and 71?

In an ironic twist of fate, I did something similar to generate smooth terrain from an array of 2d points just the other day :p

plug: https://www.roblox.com/games/1121503247/lol-kek

Try separating your functions:

local calcy = function(x, z)
	return (x^2)+(z^2);
end

local graph = function(x, y, z)
	local point = Instance.new("Part", game.Workspace);
	point.Size = Vector3.new(1, 1, 1);
	point.CFrame = CFrame.new(x, y, z);
end

graph(1, math.clamp(calcy(1, 2), -71, 71), 2);

If the name of the game is to only graph points within a certain limit, there are two good ways of doing it.

The best way would be to alter the graph() function as such:

local graph = function(x, y, z, min, max)
	if((y >= min) and (y <= max))then
		local point = Instance.new("Part", game.Workspace);
		point.Size = Vector3.new(1, 1, 1);
		point.CFrame = CFrame.new(x, y, z);
	end
end

The other option would be to do this:

local points = {};

-- normal body, but insert all instanced parts into table of points, then destroy any points out of bounds

graph(1, math.clamp(calcy(1, 2), -72, 72), 2);
for a, x in pairs(points) do
	if((x.Position.Y > 71) or (x.Position.Y < -71))then
		x:Destroy();
	end
end