TikZ

TikZ is a LaTeX library for generating graphics. (Link)

Pricing

Free and open source.

Alternatives

• Inkscape. When producing complicated shapes, Inkscape is typically easier than TikZ, but the process of exporting from Inkscape after changes to the graphics can take longer.
• Draw.io.

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Creating graphics in TikZ has a steep learning curve (the PDF manual is apparently over a thousand pages), but for simple use cases, such as plotting a function, creating plots in TikZ can be convenient since changes to the graphic can be incorporated simply by re-compiling the LaTeX document.

TikZ also comes with a substantial collection of built-in functions and libraries for generating specific types of diagrams, including

• Three-dimensional diagrams
• Angle markers
• Automata and Finite State Machine
• Electrical Circuits
• (Discrete) graphs
• Trees

For plotting mathematical functions with nicely formatted axes, you can use the pfgplots package.

If you are trying to plot something “from scratch”, try looking for a library first!