What is the derivation of the statistical term "Histogram"?

There are conflicting definitions of origin. Unclear as to whether it is derived directly from the Greek, coined by Pearson, or used and named prior to Pearson.

Refer to "The History of Histograms - 1. Prehistory" - Yannis Ioannidis

The etymology of the word histogram is uncertain. Sometimes it is said to be derived from the Greek histos 'anything set upright' (as the masts of a ship, the bar of a loom, or the vertical bars of a histogram); and gramma 'drawing, record, writing'. It is also said that Karl Pearson, who introduced the term in 1895, derived the name from "historical diagram".

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According to the Oxford History of Statistics, the word histogram was coined by Pearson, as your link also mentions. It certainly did not exist in Ancient Greek.

The Oxford English Dictionary gives the following as its first quote:

1891 in E. S. Pearson, Karl Pearson (1938) 143: "The geometrical representation of statistics. ... D. By Columns. Histograms. Optical advantage of vertical over horizontal columns."

The OED also gives the same etymology as Kiamlaluno's NOAD does: from Greek ἱστός (/histos/), mast, upright beam. This and the fact that Pearson himself put "Histograms" as his primary entry under "By Columns" as in the quote above make it near certain that he created this word based on the likeness of masts or beams to the columns of a histogram.

Note that this is not at all related to the word history: histos is cognate with our words stand, status (through Latin), and stasis (Greek), while historia is cognate with our words wise, wit, vision (through Latin), and idea (Greek).

The NOAD reports that the origin of histogram is late 19th century, from the Greek ιστός (mast, web) plus the suffix -gram.