Add plot of function

[sverhoeven]

As graphically oriented person it would help me if the function we are trying to find a root for can be plotted. That way I can better understand why the answer is -1.

For example embed something like https://www.desmos.com/calculator/yj0ih4ml0i

[sverhoeven]

We can use docsify embedding to embed the interactive plot for example with

[Plotted](https://www.desmos.com/calculator/hfb3rvqouv?embed ':include :type=iframe')

[fdiblen]

Is generating the plot using some JavaScript library an option or we really want to use an external service?

[sverhoeven]

No using a svg would also be fine.