Currently only choice for Japan

Navionics has temporary dropped Japanese charts by requests from Japanese Coast Guard.
So that this app, which uses CMAP, is the only choice for Japanese users.
This app has better screen design and visibility than Navionics and iNavX and very simple.
It doesnt support external NMEA input as same as Navionics, but can be viewer of Standard Horizons chart plotter.

atinoue about C-MAP Plan2Nav, v1.3.1