  A method for estimating the spectral gap along with higher eigenvalues of
nonequilateral quantum graphs has been introduced by Amini and Cohen-Steiner
recently: it is based on a new transference principle between discrete and
continuous models of a graph. We elaborate on it by developing a more general
transference principle and by proposing alternative ways of applying it. To
illustrate our findings, we present several spectral estimates on planar metric
graphs that are oftentimes sharper than those obtained by isoperimetric
inequalities and further previously known methods.
