(1) the user is typing at a textarea and, at some time, he types the code of the data-series (maybe, if the code is GDP, he needs to type @GDP or something like that);
(2) at the time the user finishes typing, an AJAX call is made to the server, which sends the data in some format, maybe JSON;
(3) the page receives the data and builds a chart using some pre-prepared script (with Highcharts, maybe) and puts it besides the textarea;
(4) after that, the user has the option to customize the chart (shorten the time-period of the data or add another series to the same chart);
(5) when the user submit the form, along with the text in the textarea, the customized chart must be saved in a way that allows the server to redraw it (you can create some protocol, like a JSON string which says the chart must be generated from the date a to date b with the series f and g).
If you have other ideas on how to execute this or any doubt, we can talk and clarify everything.