As I see it, you have a problem convincing people of the value of your service, particularly the value over a different solution (i.e. Excel). I like your Video on the Blog page and I'd move this to the first page. However, a scatter graph can easily be drawn in Excel - maybe add a more "awesome" example?

I don't know what you think of the freemium model, but it could be an alternative to your free trail. It could make sense to introduce a free product category with maybe 10 free graphs and datasets with less than n elements. This way, customers aren't bound by the 30 days and have more time to warm to your product.

You write that there are differences in the design themes across the different products (basic, designer, premium)? I'd explain the difference in detail - you could use a check list of the available features - so that it's clear why premium is better.