In The Solar House I discuss how modern architects developed a new understanding of the science of solar geometry through drawings. Studies including solar angles, to determine the spacing of buildings for heliotherapeutic considerations, came out of the Bauhaus in c.1930. I conclude that the first solar geometry studies made specifically for solar heating (and shading to prevent overheating) were made in late 1937, probably independently, by Keck & Keck and Henry N. Wright. In addition to those examples, Le Corbusier made such a drawing 1938, as I discuss and show in Le Corbusier and the Sun.
It's possible that Alvar Aalto should be included in that short list. In a 2012 lecture, British historian Dean Hawkes mentions that he found section drawings of the Villa Mairea with a critical sun angle of 52˚ shown. This indeed corresponds to noon at the summer solstice in Noormarkku, Finland. Aalto designed the Villa Mairea in 1938. (Video here; the pertinent bit starts about 18:42.) Hawkes is one of my favorite historians, and the full video is well worth the time.
We may never know definitively who drew the first 2D shading diagram, but clearly the convention came of age in 1937-38.