Is it more common to refer to subplots as a "frame" or a "panel"?
I can't recall having seen the word frame or panel used to refer to a subplot. More commonly:

In the text, e.g.,


  Figure 1(a) shows [...]; figure 1(b), instead, shows [...].


In the caption, e.g.,


  Figure 1. Comparison among blah blah: (a) function f [...]; (b) function g [...]