* Change templateCaptureValue to support using template functions
This commit changes templateCaptureValue to use float64 for the value
instead of *float64. This change means that annotations and labels can
use the float64 value with functions such as printf and avoid having to
check for nil. It also means that absent values are now printed as 0.
* Use math.NaN() instead of 0 for absent value
5caf6cb369