  The COMPASS experiment at CERN has collected a large dataset of $50$ million
$\pi^-\pi^+\pi^-$ events produced diffractively from a proton target using a
$190\,\mathrm{GeV}/c$ pion beam. The partial-wave analysis (PWA) of these
high-precision data reveals previously unseen details but is limited in parts
by systematic effects. The PWA is based on the isobar model, in which
multi-particle decays are described as a chain of subsequent two-body decays.
Here, fixed mass distributions for the appearing intermediate resonances, the
so-called isobars, are assumed. These shapes, which e.g. may be parametrized by
Breit-Wigner amplitudes, represent prior knowledge that has to be put into the
analysis model and may therefore introduce a model dependence, thus increasing
systematic uncertainties. We present a novel method, which allows to extract
isobar amplitudes directly from the data in a more model-independent way. As a
first application, diffractively produced $\pi^-\pi^+\pi^-$ events are
analyzed. Here, the focus lies in particular on the scalar $\pi^+\pi^-$
subsystem, where in a previous analysis a signal for a new axial-vector state
$a_1(1420)$ was found in the $f_0(980)\pi$ decay mode.
