Would it be possible to publish a journal paper about data-visualization website?
This would entirely depend on the contribution your website and techniques involved add to the state of the art. Is any element of it new and novel? I.e. something done nowhere else. It may be worthy of a conference paper for example, if he contribution is weaker.

What is your supervisors opinion? Have they hinted at this possibility?

Edit: The company's data may also feature as an issue during publication. Would they be happy for it to be used and if not can you describe your work well enough without it? i.e. if you use a figure in your paper that shows their data, such as a screenshot.