For anyone serious about a career in data visualization, it's important to start learning a new tool. In short, Excel is not the only means of representing data. There is plenty of powerful data visualization software that is available for free, including Google Docs and Datawrapper. Experiment with a couple of these until you're comfortable with how they work and what they can do. Brush off that dusty statistics book. With data rapidly becoming the lingua franca of business, a basic understanding of probability and statistics certainly won't hurt. Also, play around with numbers and practice looking at huge sets of figures and extracting a meaning from them that you can then represent visually. An amazing variety of data gets published online by government agencies and others.
Design is a popular topic these days, especially in data visualization, where it's essential to making information accessible. Being able to speak intelligently about design, and showing that you know how to use it, puts you at an advantage over pure quantitative analysts. Once you've become adept at presenting figures in visual terms, you'll find that not everyone in an organization wants to accept what the data are saying, especially if they don't like numbers or resist basing decisions on them. So you have to know how to tell a story, and how to sell what your data visualization is showing. This often requires thinking about your audience when it comes to choosing how much detail you want to share with them.