How can I hide values on dygraphs?

In my dygraphs graph, I have data for three items, thus, three lines on the graph. But one of the lines did not start until sometime after the first two, so I want the graph to be blank for old dates for that line. If I set the previous values to blank or zero, I get a weird line coming up from zero when the line starts as in the image below. Is there a way to specify a start date for just one of three series, or to set the values to some value that will not graph?

I found a simple fix. Since dygraphs is very particular about numeric values, you just set the unwanted items to null, rather than blank or zero.