This cartoon appears in the paper of the state capital, Montgomery, when Atticus is away for two weeks dealing with matters of 'emergency legislature'. It is the time of the Depression and there are mounting social and economic problems that Atticus has to help the state legislature deal with. Scout observes that 'these events were remote from the world of Jem and me.'
Atticus, then, has important work to do outside of his native town of Maycomb, although it is rather a different kind of work from the more dramatic events associated with Tom Robinson's trial. This is what the political cartoon is all about. It shows Atticus 'chained to a desk' and 'diligently writing', which illustrates how dedicated he is to his work, even if it may be 'dry' (to quote Jem) and boring: dealing with taxes, and so on.
The cartoon shows that Atticus is committed to even the dullest parts of his job, and that he is not distracted by more 'frivolous' pursuits (he pays no attention to the girls calling him). The cartoon also depicts him in a rather ridiculous light by presenting him 'barefooted' and 'in short pants' as though he were a boy. It seems to be playing on the image of the serious studious schoolboy who refuses to take part in the more lighthearted, fun-loving activities of his peers.