  We present our model-independent and data-driven method to describe
pseudoscalar meson transition form factors in the space- and (low-energy)
time-like regions. The method is general and conforms a toolkit applicable to
any other form factor, of one and two variables, with the potential to include
both high- and low-energy QCD constraints altogether. The method makes use of
analyticity and unitary properties of form factors, it is simple, systematic
and can be improved upon by including new data. In the present discussion, the
method is used to show the impact of experimental data for precision
calculations in the low-energy sector of the Standard Model. In particular, due
to its relevance for New Physics searches, we have considered the hadronic
light-by-light scattering contribution to the anomalous magnetic moment of the
muon (the pseudoscalar exchange contribution), the pseudoscalar decays into
lepton pairs, and the determination of the mixing parameters of the $\eta$ and
$\eta'$ system. For all of them we provide the most updated results in a
data-driven manner.
