In other words, if we start at -90 degrees the final longitude should be 270 degrees. If you are plotting all the way around the globe, you must have XDATA extend 360 degrees past the starting point. If you want to plot data, you must use the keywords XDATA and YDATA for the longitude and latitude data points. In addition to just making maps, the code below is set up to also overplot data into these data spaces. The first map goes from the starting longitude to 180 degrees West, while the second map goes from 180 degrees East to the starting longitude. To get around this, one map is split up into two separate maps.
This can be useful because the standard map in IDL is limited to +/- 180 degrees and data spaces get strange if you try to go beyond those limits. For this case, we are plotting the globe a full 360 degrees from an arbitrary starting point.
This help article is meant to be a longer example example for how this can be done with two maps. One of the really nice things about IDL's function graphics is that you have the ability to go crazy customizing plots and maps to make some cool visualizations.
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