Mobile health (mHealth) applications rely on personal data visualisations to promote user adoption and engagement. To ensure the success of these applications, it is crucial to understand user requirements when utilising these visualisations. This study investigates the needs and challenges of mHealth app users with a focus on data visualisations. This paper presents the results of a user survey with 56 participants, which found curiosity to be the top motivation behind using health-tracking apps. 51% of the participants had to use more than one mHealth app to achieve their health tracking goals, which usually come with inconsistency and duplication of data entry. Bar and pie charts were the most preferred forms for viewing health data. Users do not want to use complex charts. Furthermore, users preferred a combination of text summaries and charts to make sure they correctly interpret the charts. On the other hand, users face challenges related to improper information presentation, inaccurate touch interface, and an overwhelming amount of data. Customisable and readable charts encouraged frequent app usage. The implications of these findings confirms the gap we identified in existing data visualisation design and development guidelines. We have developed a set of data visualisation guidelines to address these challenges, and currently conducting evaluations on designers and end users.