Before you can visualize your data, you need to make it usable! OpenRefine is a powerful tool for working with messy data: cleaning it; transforming it from one format into another; and extending it with web services and external data.
Voyant Tools is a web-based text reading and analysis environment. It is a scholarly project that is designed to facilitate reading and interpretive practices for digital humanities students and scholars as well as for the general public.
R is a free software environment for statistical computing and graphics. The R community has built many packages that are powerful tools for visualization and text analysis. The community is also very active in providing documentation and resources.
Prior to visualizing text or data, it may be necessary to retrieve or normalize your source. If a researcher or humanist is using the command line to do so, sourcecaster may be a resource for working through common challenges.