Depending on what functionality you want to have, another possibility could be to have your application to output a text file that is HTML5 + canvas + JavaScript, like here: HTML Canvas In fact, I bet a lot of functionality of pgplot could be translated into HTML5/canvas/JavaScript output. Then your web browser can display your output.
Of course, to make this interactive (send UI events from the web browser to the app and have app change its output based on these events), one would need to integrate HTTP server with WebSockets support into the app. Such libraries do exist in C/C++, but doing end-to-end integration would be a lot of work. (Looks like a fun project, though, if only I had time…)