There are two aspects to this question.
First, there is the effect of using a third-party tool on copyrights. The nice thing is that the copyright license of tools don't affect the possible copyright licenses of the output. This also means, for example, that Microsoft has no say in how you license the documents you write with MS Word.
Secondly, there is the question if a graph representation of scientific data is subject to copyright protection at all.
Copyright exists to protect the original expression of ideas. Simple facts are not protected by copyright because they don't represent ideas. Also, mechanical transformations are not separately protected but they retain the copyright protection of the original.
Based on that, the copyright protection of your graph depends on if the underlying data is protected by copyright and if there was an amount of creativity that went into the creation of the graph. And I have some doubts if the underlying data is subject to copyright protection.
So, in the end, If it took creativity/thinking to create your graph, then its copyright belongs to you. Otherwise, I believe that your graph is not subject to copyright at all.