Wow, what an excellent site, thanks Romy! It seems like they take datasets, process them, and provide an API that allows one to query the dataset, and have like 21,000 datasets:
https://opendata.socrata.com/browse
Many of the datasets are raw, i.e. comprised of rows and columns of data, so they should be easy to pull via a REST call and then do whatever kind of visualization and analysis you'd like within objective C.
Do you have any specific datasets in mind? Machine learning meetups have restarted and we're going to meet next some time in January. It could be a good opportunity for you to find people to work on your app with!
mike