1. An apparatus for logically processing a composite graph in a formatted document, comprising a computer processor and a computer readable storage medium which stores a plurality of computer-executable instructions, wherein the computer-executable instructions, when being executed by the computer processor, cause the computer processor to: extract a composite graph block in the formatted document; parse the formatted document to obtain a text element included therein; extract a cutline element from the text element; detect correlativity between the composite graph block and the extracted cutline element, wherein the correlativity detection comprises: determining the number of in-text-illustration composite graphs included in the composite graph block; in the case that the composite graph block only contains one in-text-illustration composite graph, selecting a cutline element having a distance to the one in-text-illustration composite graph smaller than a preset distance, and making the selected cutline element as a cutline element relating to the one in-text-illustration composite graph; and in the case that the composite graph block contains multiple in-text-illustration composite graphs, making the multiple in-text-illustration composite graphs and all of the parsed cutline elements as a vertex of a bipartite graph respectively, so as to use the bipartite graph to determine the correlativity between the multiple in-text-illustration composite graphs and the cutline elements; and store the detected correlativity.