QtiPlot is fully scriptable via Python, watch the video bellow for a short demonstration of how to create and fully customize a plot using a very simple Python script:
The fact that you can run Python scripts from QtiPlot gives you the possibility to use powerfull existing scientific tools,
like SciPy, SymPy
or rpy2, thus bringing unlimited data analysis power.
Here's a screenshot demonstrating how QtiPlot can become a full-featured computer algebra system (CAS), thanks to SymPy: