fix the PG plot panel

[eianlei]

Issues to fix at the PG plot

• selection of He, N2, and sum plots in combobos at Model tab, fix this
• not scaled but uses fixed scale, fix this
• plot the M-value lines properly

[eianlei]

The plot should look like Figure 1 in “Clearing Up The Confusion About “Deep Stops” by Erik C. Baker,