The current standard C++ implementation (cyl_bessel_j) is restricted to C++17 (or tr1 for GCC). GSL or Boost dependency is too much for lightweight KmPlot.

The proposed code is adopted from Boost. It is just a proposal of how Bessel functions can be implemented.

1. Compile, install, and run KmPlot.
2. Create a Cartesian plot and enter "J0(x)" for f(x).

Test plot: