I remember reading that Portuguese and Spanish are really just extremes on a continuum of a dialects. That is, if one travels slowly from (for example) Madrid to Lisbon, one would note nothing more than subtle shifts in dialect the entire way, and there is nothing significant about the shift around the area of the border. It's just that once you would reach Lisbon, the shifts have added up to a big difference from Madrid.
Firstly, is this true? Secondly, if so, what is the technical term for such a situation? And finally, is there a list of other language pairs that have this property?