Premise is similar, both contain oppressive laws taking away things people take for granted. Media Enhancement Act in Toshokan Sensou allows censorship of books whereas in Chocolate Underground, chocolate is banned in the country by health laws.
One bans chocolate, the other bans books. Where Chocolate Underground is more of a child-friendly view on a dystopian future, Toshokan is a more "grown-up" take on the same ideas of government tyranny, personal liberty, and those who would resist. Toshokan has more violence and a romance storyline and is geared more towards adults.
Chocolate Underground and Shimoneta are about a society where stuff that people enjoy (Well, chocolate and erotica) are illegal and there are crazy laws and a government to prohibit it. There is also a underground group with crazy schemes to get around these rules. They are both very funny, but Shimoneta is much more mature.
In both of these there is something that is really important and people fight for this thing. Characters are really serious about this thing and the whole series goes around this thing. Also both series have short episodes and not very much episodes.
The main thing that is similar between Chocolate Underground and Bihada Ichizoku is the atmoshphere so even when the plots are not similar at all if you liked one of these you might still like the other one too.