bar graph vs histogram

bar graph and histogram both are nouns.

Word	Noun	Adjective	Verb	Adverb
bar graph	Yes	No	No	No
histogram	Yes	No	No	No

As nouns, histogram is a hyponym of bar graph; that is, histogram is a word with a more specific, narrower meaning than bar graph:

bar graph: a chart with bars whose lengths are proportional to quantities
histogram: a bar chart representing a frequency distribution; heights of the bars represent observed frequencies

bar graph (noun)	histogram (noun)
a chart with bars whose lengths are proportional to quantities	a bar chart representing a frequency distribution; heights of the bars represent observed frequencies