LinePlotter: don't make titles appear as t,h,i,s
In 23724934 ("plotter: Pivoted Runs") we broke the titles of LinePlotter for plots done with column=string. After that, the titles for those plots appeared as a comma-separated list of letters: l,i,k,e,t,h,i,s.
Fix it so that one-column plots display the column as the title and multi-columns plots display a comma-separated list of columns as title.