A work by Helmut Newton has the highest estimate in 'Prints & Photographs Part I' auction at Sotheby's New York on April 14. The total pre-sale estimate is $5.17m.
Data from both auction houses was used to score the sales on the HENI Auction Index.
The protest comes after the university closed the artist's show several months early.
Christie's spring sales in Hong Kong totaled $113.4m and scored a B+ on the HENI Auction Index, a third straight improvement.
Zhang's work takes the form of a visual letter to her daughter
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