Good design skills have zero correlation with good data analysis skills.
Honestly, you don't need "design skills" to make a decent graph ... just the wherewithal to think about what would make a graph clearer/easier to understand the same we expect of ourselves with writing.
I suspect there'd be a tendency to presume a design process is more like solving a problem with a clear techniques for arriving at an objectively correct solution, when in reality, it can and often if just the iterative process of assessing-adjusting-repeat. Being better at these things to the point of sometimes reducing the iterative loop to 0 repeats is "design skill". But the essential base-line ingredient, IMO, is being willing to enter the iterative cycle at least to some extent.
I’ve seen wonderful and factual data analysis presented in the most horrible default templates and hacked together screenshots.