Exercise statement says "... we need to set group = 1." I think this must refer only to the first example, which does not include the fill aesthetic. For the second example, I think the intended fix is to have group = color, in addition to having fill = color. (Here, color is a variable in the diamonds data frame, not an aesthetic.) Perhaps the intention is for the reader to figure this out on their own, but I found it a little confusing.
Exercise statement says "... we need to set group = 1." I think this must refer only to the first example, which does not include the fill aesthetic. For the second example, I think the intended fix is to have group = color, in addition to having fill = color. (Here, color is a variable in the diamonds data frame, not an aesthetic.) Perhaps the intention is for the reader to figure this out on their own, but I found it a little confusing.