TODO

* More unit tests as not all functions are being covered by tests, notably 3D plots, such as contourplots and heatmaps.
* Improve print_gnuplot_handle().
* Further refactoring.
* wxt is the default terminal type, but need a test to check whether wxWidgets is actually installed: find / -name libwx* 2>/dev/null
* Plotting styles have been added to gnuplot since this interface library was completed, hence further testing and documentation.
* Do common pitfalls lead to a graceful exit?
* More field testing is needed: 
  - Does the current version still work on MS-Windows / OS-X / other operating systems? 
  - How does it perform in high-volume or high-performance environments?
  - Are there limitations which need to be looked at, for example the maximum amount of open gnuplot files (GP_MAX_TMP_FILES)?
  - Does it still work if gnuplot is installed but qt or wxWidgets is not?