Uniformize headline: graphs and plots

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Component: **documentation**

Author: **Kwankyu Lee**

Branch/Commit: **[`693c524`](https://github.com/sagemath/sagetrac-mirror/commit/693c524b1ce0792c9ff6c1f40bb23cd69174340f)**

Reviewer: **David Coudert**

_Issue created by migration from https://trac.sagemath.org/ticket/34551_

