Wish every county web site was as clean and informative as the San Francisco one
http://www.sfelections.org/results/20160607/#english_summaryGives who the nonpartisan voters took ballots for. Gives totals for write-ins and Undervotes by party.
Undervotes and write-ins were quite high for all 3rd parties, but people taking AIP ballots are clearly in a world all their own. More than half of the votes were write-ins and on top of that 1/3 didn't even vote for any candidate on the Presidential level. Things were quite similar in Sacramento county.
The Undervote was quite high in the Republican primary in SF too (8%) and in Sacramento it was more than 5% (vs 2% in the Dem primary). Maybe other major counties provide this info in final reports, but none of the major SoCal counties have it with their current reporting.