plot_conv: Add labels with id of the function
pplot fval_by_budget(): Throw away funvals after ftarget reached
ert_by_target(): Add extra ytics for 2x, (PF/2)x slowdown
pplot rank_by_budget(): Include groupby in ylabel
plot: Instead of ert, use <groupby> (median) evals# everywhere

ERT means average evals#, but since we use median for fvals, the runs in
graphs with fvals/budget did not match with runs in graphs with
target/ert. This change avoids the mismatch and reintroduces consistency.
plot_conv: Introduce symbolic function set names + way to express set operations