Exploratory information search can challenge users in the formulation of efficacious search queries. Moreover, complex information spaces, such as those managed by Geographical Information Systems, can disorient people, making it difficult to find relevant data. In order to address these issues, we developed a session-based suggestion model that proposes concepts as a "you might also be interested in" function, by taking the user's previous queries into account. Our model can be applied to incrementally generate suggestions in interactive search. It can be used for query expansion, and in general to guide users in the exploration of possibly complex spaces of data categories. Our model is based on a concept co-occurrence graph that describes how frequently concepts are searched together in search sessions. Starting from an ontological domain representation, we generated the graph by analyzing the query log of a major search engine. Moreover, we identified clusters of ontology concepts which frequently co-occur in the sessions of the log via community detection on the graph. The evaluation of our model provided satisfactory accuracy results.