  The so-called Columbia plot summarises the order of the QCD thermal
transition as a function of the number of quark flavours and their masses.
Recently, it was demonstrated that the first-order chiral transition region, as
seen for $N_f \in [ 3,6 ]$ on coarse lattices, exhibits tricritical scaling
while extrapolating to zero on sufficiently fine lattices. Here we extend these
studies to imaginary baryon chemical potential. A similar shrinking of the
first-order region is observed with decreasing lattice spacing, which again
appears compatible with a tricritical extrapolation to zero.
