spectral tab: changing View/Plot C Function causes plot C to appear

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As we discussed this morning, when on the spectral tab, plot C should only appear if there's a dataset selected in the "Plot B" combobox. Furthermore, the plot C menu items are only meaningful when there's a dataset selected in the "Plot B" combobox.

At present there's a bug where one can make plot C appear without having a dataset loaded for B.

1. open press_cp0.xml (attached). Note that the plot area shows just one plot. So far, so good.
2. Open the menu item View/Plot C Function and note that it is set to the default of Residual A-B. This is fine -- since there's no dataset B open, these Plot C Function menu items are not meaningful.
3. Change the View/Plot C Function to Residual B-A or Sum A+B. The plot area changes to showing plots A, B and C. This is a bug; it should not change the plot.