wardley.love/0055-y_at_x

y_at_x = function(startx,starty, endx,endy, x)
	-- y at a given x on a line
	-- Could be nil when the line is parallel to the y axis.
	if startx == endx then
		return
	end
	return (endy-starty)/(endx-startx)*(x-startx) + starty
end