Allow for multiple x-coordinates in the data

[yanofsky]

For instance two series of a similar duration but different frequency, like the monthly reported US unemployment rate and the weekly reported US initial jobless claims

date	US Unemployment Rate	date	US initial Jobless claims
3/1/13	7.6	3/2/13	350500
4/1/13	7.5	3/9/13	347000
5/1/13	7.6	3/16/13	340750
6/1/13	7.6	3/23/13	343000
7/1/13	7.4	3/30/13	355000
8/1/13	7.3	4/6/13	358500
		4/13/13	362000
		4/20/13	358250
		4/27/13	343000
		5/4/13	338000
		5/11/13	340000
		5/18/13	340500
		5/25/13	348000
		6/1/13	352500
		6/8/13	345750
		6/15/13	348500
		6/22/13	346250
		6/29/13	345750
		7/6/13	351250
		7/13/13	346500
		7/20/13	345750
		7/27/13	341750
		8/3/13	336000
		8/10/13	332500
		8/17/13	330500
		8/24/13	331750
		8/31/13	328750
		9/7/13	321750
		9/14/13	314750