Gnuplot is a Command Line Interfaceutility that can create diagrams from plain text markup.

Simple graph

reset

set title "a simple graph"

set xrange [0:1]
set autoscale y
set xlabel "frequency of A"

plot x * x title 'AA', (1-x) * (1-x) title 'aa', 2 * x * (1-x) title 'Aa'

See also

Gnuplot diagrams can be used in Org Babel source blocks
PlantUML and ditaa are other plain text based diagramming tools.