the infinite line

What about the concept of "infinity" was appealing to post-modern artists and how was this concept depicted through various artworks?

How can "artworks" by Agnes Martin be considered to be art when they are simply lines of grids on a paper, with various alterations?

While I understand that there is some underlying significance on Martin's work, I fail to see the connection to art.  How can a page of lines be compared to some of the great artworks on the Renaissance?  Obviously the times have changed and the notion of art has evolved, but to equate grids to something like the Mona Lisa, or other renowned pieces, seems to slight the very definition of art