Graphs improvement

[PetrCala]

• Smooth out the bpe graphs line
• Add a normal distribution line to the t-stat distribution graph
• Add prisma fascie
• Consider displaying the bpe in histograms perhaps?

[PetrCala]

@3rd point - this should be done by constructing a whole new functionality, which will work very similarly to graphBPE (potentially consider reusing the function?), and the user will be able to specify ids of variables (factors) for which to construct histograms or density graphs for, and have the option to choose which one they will use too.

[PetrCala]

Done