14. The system of claim 10 , wherein the one or more programming instructions that, when executed, cause the computing device to receive the one or more content items comprise one or more programming instructions that, when executed, cause the computing device to: cause a set of candidate content items to be displayed at the user computing device; and receive a selection of one or more of the candidate content items.
14. The system of claim 10 , wherein the one or more programming instructions that, when executed, cause the computing device to receive the one or more content items comprise one or more programming instructions that, when executed, cause the computing device to: cause a set of candidate content items to be displayed at the user computing device; and receive a selection of one or more of the candidate content items. 15. The system of claim 14 , wherein the computer-readable storage media further comprises one or more programming instructions that, when executed, cause the computing device to: determining a position of each of the selected contents items on the graphical representation, wherein causing a three-dimensional graphical representation of the structural document to be displayed at the user computing device comprises causing the selected content items to be displayed at the corresponding position of the three-dimensional graphical representation.