Bokeh refers to how the lens, based on its design, renders the out of focus areas. Logically, no lens test chart will measure this, since they are made by focusing on a two dimensional chart. These two dimensional charts are great for sharpness and distortion, but tell us nothing of how the out-of-focus areas will look. The aspect of the lens design that influences this the most is the aperture shape. I would like to write more about what exactly causes what, but I would need to do more research first.