Am I breaking any law by digitalizing a curve from a plot on a scientific Journal if I cite the source and I mention the tool I used?
At least in the USA this is explicitly OK, so long as by 'digitize' you mean 'find a mathematical expression of the nature of the curve'. As long as you do not use the original image, you can even reproduce the curve from the that information. Data, as in observations about the natural world, cannot be copyrighted.
If you are 'digitizing' in the sense of creating a digital image, most journals will request you obtain permission to reproduce the image you found in the original article. &quot;Creative and expressive' mediums, such as data visualization, are fully copyrightable.
You also might consider reaching out to the authors anyway, to get their own data used to produce the curve. Reviewers and readers will prefer this, as it ensures the description of the curve is accurate. Almost any author should be amenable to this.