Improvements for the 2D Grace plots created by the rdc.corr_plot user function.
The autoscaling is now turned off, as the data set representing the diagonal (with points [-100, -100] and [100, 100]) causes the world view to be set to be between -100 to 100 or -200 to
200.
The world view is set to be between -50 and 50 Hz, so that all RDCs should be visible. The ticks in
the plot have also been set so that the minor ticks are at every Hz increment.
