Make addCurves method to ignore columns it shouldn't draw.

Refs #7579
Slight refactoring of the method.

Refs #7579
Specify X columns for methods.

Don't use convenience method versions, as this will allow to specify
our own X columns later on.

Refs #7579
Remove some repetitive code.

Refs #7579
If there is a single X column in the list - use it.

Refs #7579
Convenience method to get a list of selected X columns.

Refs #7579
Pass the whole selection to the method.

And check for multiple X columns selected as well.

Refs #7579
Merge remote-tracking branch 'origin/feature/7579_plot_using_selected_x'