  Charge radii measured with high accuracy provide a stringent benchmark for
characterizing nuclear structure phenomena. In this work, the systematic
evolution of charge radii for nuclei with $Z=19$-$29$ is investigated through
relativistic mean field theory with effective forces NL3, PK1, and NL3$^{*}$.
The neutron-proton ($np$) correlation around Fermi surface originated from the
unpaired neutron and proton has been taken into account tentatively in order to
reduce the overestimated odd-even staggering of charge radii. This improved
method can give an available description of charge radii across $N=28$ shell
closure. A remarkable observation is that the charge radii beyond $N=28$ shell
closure follow the similarly steep increasing trend, namely irrespective of the
number of protons in the nucleus. Especially, the latest results of charge
radii for nickel and copper isotopes can be reproduced remarkably well. Along
$N=28$ isotonic chain, the sudden increase of charge radii is weakened across
$Z=20$, but presented evidently across $Z=28$ closed shell. The abrupt changes
of charge radii across $Z=22$ are also shown along $N=32$ and $34$ isotones,
but the latter with a less slope. This seems to provide a sensitive indicator
to identify the new magicity of a nucleus with universal trend of charge radii.
