If you make a search at google.com for a mathematical equation google will in most cases automatically plot it for you.
For example, you can type in
sin(x)/x or even a comma separated list such as
sin(x)/x, cos(x), x to plot multiple functions on the same graph.
An answer to this question could be <a href="https://github.com/mauriciopoppe/function-plot" rel="nofollow">function-plot</a>.