The digital slideshow “On Line,” which borrows its title from a 1919-20 essay by Vasily Kandinsky, focuses on drawing’s most basic element. Beginning with the year 1910, and ending a century later, the survey is organized chronologically and thematically in three separate sections: surface tension (the representation of movement on a flat plane), line extension (the use of contours to advance into a third dimension), and confluence (the collapse of outlines into their backgrounds). It’s a witty art-historical tour of “line” in its various permutations. —C.J.F.