There is no way to visually show growth in a smart list or list. But you can visually show membership trend for a smart campaign. Perhaps you can set up a smart campaign for this form fill action and the flow will be to add to a static list and that will allow these people to be members of this smart campaign. You can then use the smart campaign's "Membership Trend" tab to monitor how many are being added to the smart campaign.
Although not giving you exactly what you are looking for, you can capture the # in the lead performance report. Just group people by created date and then select months (or whatever period you want to compare) and then in custom columns, add your smart list. Finally, in the smart list, add the "Member of a Smart List" filter and select the appropriate smart list so that you only see those leads. This will return a table of information that can easily be exported and turned into a graph.