I'm trying to learn about the geometric interpretation of NMF. I have found the paper by Slim Essid to be very useful. I would like to make a plot like the one in Figure 1 just for a k=2 Topics (i.e. the 2d case). What are the axes in Figure 1. Thanks for your help.
The axes are literally dimensions in the feature space. Vectors inside are data points. Vectors between the data and the positive axes are the "topics". I think Page 22--24 (especially Figure 3.1) of Gillis' thesis (https://www.dropbox.com/s/dyfusffzymewoim/Thesis_gillis.pdf?dl=0) should be helpful for the precise definition of these things. Sec 3 of https://www.stanford.edu/~vcs/papers/NMFCDP.pdf is also very clear about the geometric interpretation of NMF (it is probably where it first appeared).