How to create Google Chart with lines (series) of different lengths?

How do I create a Google Line Chart that displays two or more lines, with a different number of data points in each series?

For instance, I want to create a chart with 2 lines, one showing the expected values over time and another showing开发者_运维技巧 the actual values over time. The date range and expected values are known in advance so I can fully graph them, but the actual values may not be fully known yet (e.g. the date range covers some dates in the future).

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I found the answer in this SO question. The solution is to use "_" (or "__", depending on the encoding of the data values) to indicate "no value".

For instance, one data series might be 10,7,3,1 and the other might be 10,6,_,_.