Valentina D'Efilippo is equally comfortable immersed in science as she is in storytelling and graphic design. That tension has shaped nearly two decades of work translating dense, unwieldy datasets into visual stories: flowers standing in for a century of wars, a dandelion holding the shape of the #MeToo movement on Twitter. A cold email from HarperCollins turned an early instinct for this kind of work into a profession—pairing her with data journalist James Ball on The Infographic History of...